View Full Version : Rs4 50

02-01-2016, 07:06 PM
Seems a bit odd admittedly, that this is a new bike, (only a little over 6 months since i picked it up from the dealer) and not a pile of parts salvaged or a rusting and battered example of a classic RS from eBay, but it was always going to become a mini project of sorts to me. So I hope you can bear with me on this and I look forward to comments and input along the way.

OK, here is what I'm starting with:


(http://uk.aprilia.com/mediaObject/aprilia/models/moto/road/RS4-50/main-image/rs4_50_nero/original/RS4_50_black.jpg)It had some minor works done at the dealership before collection (resonator tube? removed from the exhaust, jets, needle and needle valve changed, along with a 13T sprocket and a new plug), which I have to say make it run pretty well for something that displaces around 3 cubic inches.

They tell me that this was in line with Aprilia's guidance to preserve the warranty status.

Stock it has some pretty cool stuff, like the braided hoses and the huge disc brakes, good 17.5mm carb (a decent size) and I personally think it looks the part. It does go quite well, and handles great but I'm sure that there is more that can be realised.

Now here lies my problem. New bike and a 2 year warranty but I'm desperate to start adding a few custom things! At least if i forgo the warranty I have a bike that I know the history of from day one and a clean slate to work with.

I've already started to add some aftermarket components and have more waiting to be fitted (I'll list what else I've got in my next post).

So here we go.

The lack of any temperature indication was a bit of a let down. A red warning light that comes on just before it's about to explode doesn't quite cut it.

Looked around for a while at possible solutions and considered this:


but ended up with one of these:
299168 I'm thinking, how can I easily fit this and not affect the warranty. Get the dealership to fit it?

Or use these to get a temp reading,
and power it all with one of these: 299166

Result! Works like a charm, I appreciate that the temperture reading may not be quite as accurate as having something thats actually touching the coolant, but it's 100% better than the one it came with!

Stage6 unit came with some 3M/velcro to attach it and I bought some more to fix the battery unit inside the fairing. I'll take some pictures of it mounted to add here later.

Modification #1 complete!

02-02-2016, 04:25 PM
Here are the other parts i have picked up so far:

Giannelli 2T Exhaust with Carbon Silencer
Dellorto PHGB 21mm Racing Black Edition
Mallossi VL6 Reed Valve Block
AFAM 14T front sprocket
Regina Chain
A selection of Jets (Main & Idle) for the PHBG

Was a bit unsure after getting the 21mm carburettor and felt i may have shot a bit high with this. I always had one eye on the future and thought it would be well suited to a BBK of some sort later. Should I have gone with the 19mm?

I've seen in various threads that a 21mm can be tuned to suit a 50cc (just!), and seen charts which show this to be right on the limit. Will probably need to get even more jets and perhaps a needle or two.

Thanks to XERO's recent project posts on fitting the same carb I have an idea on what may be necessary to squeeze this into what looks a pretty tight space and I'm definitely envious of that garage!

Stock it's fitted with a 17.5mm PHVA so may just try and up-jet this at the moment? There are a few other things i will need to consider with the new carb including what looks like heating tubes running into the PHVA.

I can relate to what XERO said in his project thread:

I purchased my RS knowing full well what it was and what (not) to expect out of it.
If I wanted real Sport Bike performance I would have just went out and picked up another Sport Bike.
So again, I wondered why would I rip the heart out from this little Italian bike to make it into something that it isn't ? (nothing against those that have)
Finally I committed, my goal was to see just how far one could take this single cylinder 2 stroke engine.
I must point out that I am not looking to turn the engine into a grenade, I want it to be reliable and still function as a viable "street bike".

Here's hoping i can achieve the same.

02-02-2016, 07:26 PM
Many of us have started our projects with new bikes. They come prepped for 16 year old beginners. To get them where we want them takes an extra two or three thousand dollars (or a lot more). :lol: It's all part of the fun. ;)

It's very easy to get over $10K in to these bikes. Especially if you start to mess with brakes and suspension. The trouble happens when some cager runs over your bike, and you have to explain to the insurance company that thinks your bike is only worth $200, and why you believe it's worth $10K. Let me tell you from experience--this is a difficult task, because they are professionals with lots of tricks up their proverbial sleeves, and most of us come at this from pure ignorance.

If I were to suggest one item that folks should start putting on these hopped up bikes religiously it would be an EGT gauge. I've written some pretty extensive stuff about them in my Wicked thread (http://www.apriliaforum.com/forums/showthread.php?312028-Derbi-Reactivation-Project-The-Return-of-the-Wicked-GPR). This is much better than a head temp on the plug (which is much better than nothing). But head temp on the plug will tell you too late if you are about to fry your engine. But used with an EGT you can better tune the engine, and save it from blowing up.

Another thing to learn about, and employ are leak down test kits. Make up one of these for your bike, and use it when you do a build, or are having trouble with the engine. Matt swears by these, and I've become a believer. ;) Again, written about in the Wicked thread. ;)

I've got that same carb and have put a lot of miles on it--although now in is retired, and sitting in a box. I bought the red one for someone as a gift, and he was never able to figure out how to tune it. It's a tricky carb. The Polini CP seems to provide similar performance with much easier tunability. If you are going to go to what they call the hyper kits (80cc plus) then look at the PWK for it's designed for the larger kits.

If you are running the stock kit with a 21mm carb you are going to learn a lot about tuning. You can get more power, but it will be much more finicky, affected more by conditions, and have less bottom end. But it will within a narrow range develop more power--but not top speed.

I wouldn't waste your money on a new chain and sprockets. The stock stuff is pretty good, and you'll not be wearing that out any time soon. Unless you abuse it by not keeping it cleaned and lubed. For sprockets--the front sprocket has the same dimensions as a Honda CR80 (not 85). You should be able to find these for $5. Also ditch the attachment mechanism, and get an E clip. Way better system, and allows for easy sprocket changing.

The head on these bikes has a temp sensor. You can get one that is calibrated to your gauge and thus have a sensor on the water temp. Which is better than a spark plug temp sensor. I've written that up in one of my three current threads (don't remember which one at the moment!). :lol:

Lots of companies make decent manifolds for these bikes, and the various carbs. You can modify the stock manifold (with a rotary tool) to work with the carbs, but it's better to just get one designed for it. Molossi makes one for the 21mm carbs that comes with those reeds. They are pretty nice, but there are less expensive ones too. There are lots of advantages to doing it this way--quicker throttle response being one of the best ones. :)

Oh and you should know, with regards to your engine. Think of it as a Derbi. With regards to the frame you can think of it as the classic RS, but with some changes. With regards to the brakes, and front suspension think of it as a Derbi. This bike may say Aprilia on it, but it is more Derbi than Aprilia (just like the MotoGP 125 bikes). ;)

02-02-2016, 07:32 PM
Welcome, and thanks for the complements.

I have no personal knowledge in respect to the RS4, but general advise is always available.
There is a lot of current (posting) Members with a lot of great collective knowledge and experience to pull from here.
There are also some Members with more opinions then experience ...

A 21mm Dellorto is completely tune-able to a 50cc barrel providing you have everything in order and are willing to invest the time and patience needed to get it dialed in.

If the RS4 is anything like the RS50 in regards to the "heating tubes", they can be plugged at their source and eliminated.

As for your concerns about your Warranty, do you guys have the "can of worms" saying there in the UK ???

02-02-2016, 07:38 PM
With regards to your warranty. Keep ALL of your stock bits. If something happens remove ALL of the hop up parts, replace ALL of the stock parts. Bring it to the dealer, and say only these magic words "I woke up and I found it this way." After that keep your mouth shut. Most warranties are voided by spewage from the owners mouth. Just keep saying "I don't know," and deny anything they might accuse you of doing. Never admit to having done any mods to the bike--never!

Yeah this sounds like BS, but remember they will use these things against you, even if they didn't contribute to an inherent issue with the bike. So keep your mouth shut. ;)

02-02-2016, 07:45 PM
Bring it to the dealer, and say only these magic words "I woke up and I found it this way." After that keep your mouth shut. Most warranties are voided by spewage from the owners mouth. Just keep saying "I don't know," and deny anything they might accuse you of doing. Never admit to having done any mods to the bike--never!

LOL !!!!!!!!!!!!!

Serious words of wisdom there :plus::plus::plus:

Clint Eastwood
02-02-2016, 08:29 PM
ditch the attachment mechanism, and get an E clip. Way better system.299288

E clip. (very unsafe system), allows the risk of sprocket flying off at high speed - snapping the chain/destroy sprocket cover & or riders left ankle/foot. Same type of reason front/rear wheel spindle bolts use a nyloc nut - some bikes even use a hair clip shaped pin through the bolt as an extra safety measure if the nut or sprocket retaining bolts come loose.

02-02-2016, 08:43 PM
Ian if you want to state your opinion on things that's fine. But stating that I said those things is out of line.

Clint Eastwood
02-02-2016, 09:08 PM
stating that I said those things is out of line.299301

02-03-2016, 05:40 PM
Thanks XERO, good to know that the 21mm is do-able from your experience with the stock 50cc. Reading through a lot of the threads I see that the Dellorto can be difficult to setup and would be a labour of love in its own right. One of the reasons I went for the black racing model was the screw top and drain nut on the float bowl so as to make it easier to change needles and the main jets.

Thought I could maybe just loop the 2 outlets from the main cooling pipes together if I couldn't figure a way of sealing them off effectively.

ToraTora glad to hear that I'm not the only one, the money spending has well and truly started. I'm sure that some will think it's crazy to spend on something that will still only ever be a small cc'd 2stroke, but to me thats the beauty of it, as I'm sure it is to many of the enthusiasts here.

I've started looking through your Wicked thread, and the EGT is something that I had looked at. Saw a Stage6 one for around the 80 mark, but there are all sorts of sensors etc that need to be setup and it all looked a bit daunting! Having a lambda or EGT would be nice though.

I hear what you're saying with regard to the chain and I do lube it, its just that the back wheel is already almost at the stops. I guess when they put on the 13T fromt sprocket this took up all the slack. I wanted to try the 14T when I fit the Giannelli so as to not over-rev the engine and figured it might be able to pull this ok. But I'm not sure that the chain would even reach correctly with the extra tooth on the sprocket so got the chain to be sure that it would work.

When I got the temp guage I did buy a head sensor (as well as the plug one), couldn't find much info on the thread size although they said it would fit all Piaggio LC heads. Was a little worried about fitting it and how much coolant may start spewing out, so took the easy option and just fitted the spark plug sensor for a quick win.

02-03-2016, 06:05 PM
I said I would post photos of the temperature guage fitted and here they are. Apolgies, they are not that great and a little purple round the edges but they were shot on my HTC phone and it's not that great in low light.


and one a little closer.


Was so easy to fit using the plug sensors and the 9v battery box puts out just enough juice to run it. Would be nice to wire it in to the ignition at some point, but the battery box has a little switch and is accessible enough inside the left fairing. so until I need to change this I figure this is more than sufficient and does what I wanted it to do.

02-03-2016, 07:47 PM
ToraTora would i definitely be needing a new inlet manifold? I'm not too keen to start carving chunks off things though. I see you had a Malossi, I've also seen a Polini one that is of a similar construction to that. Stage6 also do a short inlet that may fit in with what XERO was saying about the increased size of the 21mm carb and needing a little more space.

Oh, I seriously like the look of the Brembo master cylinder levers. Sweeeet.

02-03-2016, 10:31 PM
Ian I don't where you are getting your information. Race bikes wouldn't make it through tech inspection (which they must go through at every race) if the E clip was unsafe.



You can make the stock manifold work with the Del sized carbs (this includes the CP). But like I said there are issues, and there are advantages to employing an intake manifold optimized for the application. The ones you mentioned are all good ones.

From the reading I've been doing with the oxygen sensors there are reasons why they are trouble on strokers. Apparently they get clogged up quickly (and are then ruined). There is that gadget that supposedly helps, and I'm probably going to try it at some point. The EGT gauges on the other hand do work. The inexpensive one looks worthy of a test. I will probably try this first. The moped cats have them working. ;) The Stage 6 unit is pretty nice--and I almost bought that one (but the shop that had it wouldn't give me a discount, and well I don't like it when they do things like that...). And of course things like the Starlane Corsera are stunning. I've got to find it again, but when watching the water/head temp in conjunction with the EGT you can tell a lot about the engine. Matt already has some of this working on his bike, so I think he'll be the one to educate us in this area. ;) I do know from experience that the head temp sensors wont tell you fast enough to save you from a seize.

If you upgrade your caliper to the four piston then get the RCS15. If you run the two piston get the RCS14. ;) There's nothing like brakes with great modulation. :) I have the 15 on the Black bike, and I'm probably going to go with the 14 on the Silver bike--but then the nice Domino switch I just picked up will have to go on the inside of the Brembo because the Brembo wants to be right next to the throttle. Or so it seems.

On my GPRs I've been able to go with front sprocket changes on a stock chain from 12 to 15 without a problem. Could be different with the RS4. I don't think by just adding the pipe and carb to a stock 50cc top end (without port mods) is going to be enough power to pull a 14 that strong. Your top speed will improve with the taller gearing, but you'll notice it on the take offs. But do try it, that's how you learn. I tend to run a 15 on the front with my kitted bikes, unless I'm going to tighter corners then I'll put on the 14. My friends run 12s on the track--I'm probably going to be doing that soon too. ;) The main reason for that is to not have to shift into first gear after a start.

Clint Eastwood
02-03-2016, 10:41 PM
Ian I don't where you are getting your information. Race bikes wouldn't make it through tech inspection (which they must go through at every race) if the E clip was unsafe.

http://i1064.photobucket.com/albums/u365/derbigpr/bikes/buddys%20bikes/MTTB/IMG_3265_zpstju442bo.jpgKnew someone would bring that up so here's an explanation: track days & MOT road rules differ in certain regions, as a metaphor' the drive/spur gear E clip on 1 of my 1/5th scale rc cars always flew off. This is why a pair of bolts & or nyloc nuts are (much safer). its common sense - "Fact!" :lol:

02-04-2016, 11:36 PM
From the reading I've been doing with the oxygen sensors there are reasons why they are trouble on strokers. Apparently they get clogged up quickly (and are then ruined). There is that gadget that supposedly helps, and I'm probably going to try it at some point.

I have done a bunch of tuning on autos with "stand alone" A/F Meters and I can attest to them being sensitive to oils, excess fuel, and moisture in the exhaust.
O2 sensors have "heaters" built into them to get the little platinum elements inside up to about 600 Celsius.
They need to be in that heat range to be able to properly ionize / analyze the gases (looking for O2 content) in the exhaust.
If they get covered in liquids they can break inside (thermal shock) or get clogged-up by things like oil.

Considering that 2-Strokes typically spew excess un-burnt oil I can see how that would greatly reduce their effective lifespan if used.
One must also take into consideration that the addition of 2-Stroke oil effectively Leans out the A/F ratio produced by the carb's jetting.
To further the 2-Stroke variables we have to also then consider the type of 2T oil being used, Standard vs. Synthetic.
2T oils work / burn differently depending on Brand and composition and in turn requires slightly different "tuned" A/F ratios from the carb.

We must also also look at the fact that a carburetor is a "blunt" instrument for fuel delivery.
It's specific A/F at any given Throttle position can be altered by all sorts of things like load, temperature, porting, environmental conditions, etc. ...
EFI on the other hand has the ability to adapt to constantly varying conditions and regulate the fuel delivery for the programmed Target A/F Ratio.

When talking about O2 Sensors we must also take into account what type of O2 Sensor we are referring to, Narrow or Wide Band.
Narrow Band O2 Sensors will vary their voltage flux in a very narrow region relative to Stoichiometric (14.7:1), this is ok if you are within the realms of a legitimate combustion process.
But, 14.7:1 does not produce "Peak HP" so if all the Gauge is giving you is a colored bar graph you possibly will never be running in the "Green".
Wide Band O2 Sensors have a greater range of A/F readings and can be more useful during an initial set-up and for fine tuning.
They also offer a "bench mark" comparison which is more accurate , meaning they are technically 2 Narrow Bands in one.
One measuring atmospheric the other measuring exhaust O2 content.

I have also found that Timing and A/F go together and just because you are running a specific or recommended A/F Ratio doesn't mean that you will be getting maximum performance given your current timing settings / curves.

I haven't done any tuning on a 2-Stroke with an A/F Meter so I can't really say anything for certain, but can offer up my experience relative to tuning done on 4-Stroke Auto Engines.

So to sum that all up, I wouldn't waste my money on any type of A/F Gauge for daily use, as Tora pointed out an EGT would be much better for a "real time" assessment of what's going on.

02-05-2016, 02:27 AM
I totally agree with all of that! This gadget from Koso (http://www.snowparthub.com/BG001855_Koso_North_America_O2_Sensor_Bung_Adapter _for_2_Stroke_Engines?src=Google&gclid=Cj0KEQiAisy1BRD7_YSgpduD2cEBEiQAPR3UuIyssGTD sDScHvfz-6aFWQ8E1kndLnxwoOQea9-QnXYaAp198P8HAQ) is what makes me think that hmmm...

Apparently you pop this little "bung" on the end of the wide band oxygen sensor, and it protects it from the oils. You wouldn't run it all the time, just long enough to get the tuning setup. So within these parameters it might be functional.

Now the real reason why you would want to run an oxygen sensor all the time would be if you were doing fuel injection (especially direct injection) so that you would have real time (or close to it) feed back, and be able to regulate the parameters. I am no where near that point in my tuning. But I'm starting to think about it. :lol:


02-06-2016, 04:21 PM
Splashed out on one of these. Well constructed and when properly fitted is rock solid. Heard a few horror stories about cheap paddock stands and didn't want to take any chances with my investment. Knowing my luck the unthinkable would probably have happened, hopefully this will hold up as well as the manufacturer claims.


They also claim one person easy operation and I think I would concur.

02-06-2016, 04:24 PM
From what I've read the EGT sensor needs to be within a certain distance from the piston. Some claim no welding necessary. How does that work? Self tapping?

02-06-2016, 05:09 PM
They use what is basically a hose clamp to hold it in. I would have it welded in for anything other than a quick test. The sensor itself isn't welded in, but a threaded insert into which you thread the sensor. ;)

02-07-2016, 12:02 AM
How's that stand mount? Just to the axle bolts?
love these bikes, really wish I would have bought a real rs4 but oh well maybe in a couple years!

also just for inspiration
http://i51.photobucket.com/albums/f370/Dawntherockets/Aprilia-RS4-125-Tuning-6_zps2dquztr2.jpg (http://s51.photobucket.com/user/Dawntherockets/media/Aprilia-RS4-125-Tuning-6_zps2dquztr2.jpg.html)

02-07-2016, 05:07 AM
Dcrac3r it goes over the swingarm bolts toward the centre of the bike. Here is their site http://www.abbastands.co.uk it has some videos showing the fitting etc. It's about twice the price of an average quality regular paddock stand but well worth it I think.


02-09-2016, 02:02 AM
yo can ya'll just ship all these rs4's and all the last batch of two stroke rs125's to us over in the states in a really bigass shipping container plz plz plz plzzzz. /jealousy.

J/k good luck with your build dude i bet that thing is going to rip like a dream!

02-09-2016, 01:30 PM
Molino, :). There doesn't seem to be that many of the last released 2 stroke's available even over here. Was looking today and could only see 2 late models for sale. One from 2011 and the other from 2013. To be honest if i had the spare 3000, I would probably buy one! Although maybe those with them know what they've got and are reluctant to part with them. Truly the end of an amazing motorcycling era, such a shame that they now favour the 4 stroke engine. Partly the reason I chose the RS4 50 was because its still a 2 stroke engine.

Thanks for the good luck message I'm looking forward to progressing with it. Stay tuned.

02-09-2016, 01:42 PM
Thinking of one of these rear seat cowls too.


They are a bit expensive, but with the passenger footpegs removed would look pretty nice.

02-12-2016, 10:43 PM
Sebimoto sells a really nice fiberglass tail btw

02-13-2016, 02:14 AM
Sebi makes a lot of cool parts (http://www.sebimoto.com/aprilia-rs4-125).


It's amazing that you can order this fender for 23! I had to pay over $100 to get a fiberglass fender for the older GPR from the folks in San Diego. And you can get it in carbon too. This is the same shape as the fender I bought from Tyga for my GPR.


02-13-2016, 06:54 AM
Have had a look at this, definitely look good and nice to be able to get carbon an kevlar options although this does hike up the price a bit.

For the black option it's another 20%, so around 55 plus the shipping. Then add the matte paint (and maybe replacement decals) and I could be getting close to the cost of the original part.

If the fairing had been damaged at all then this would be an excellent and cost effective way to replace the whole tail end. Certainly going to bookmark this for future reference.

May give the dealer a ring for a solid quote on it. So far I've just been pricing it up from an online parts retailer. I'll post the quote when I get it from them.

02-15-2016, 02:59 PM
After speaking with the dealer, it's ordered as parts not a complete unit. Priced up the individual parts online and they come to around 125. Steep! AF1 are advertising one for the RSV4 for about $80 (http://www.af1racing.com/store/Scripts/prodView.asp?idproduct=56055). Surprisingly to me they seem to be exacly the same all bar the cushion. I've emailed them to see if the correct RS4 cushion could be substituted in the advertised kit. Even with the shipping to the UK this could be a cheaper option. However, with the Import taxes it would make this a little over 100. My local dealer is emailing me a quote and availabilty.

02-15-2016, 04:00 PM
You should post the part numbers for future rs4 enthusiasts ;)

02-15-2016, 04:34 PM


It would be sweet if they were the same parts. I suppose you could just travel over to the dealer and measure things out. Here's the RS4 parts diagram from Easy Parts (http://www.easyparts.nl/showdrawing/12427/170125/sbk/0-4019-1-m412-m1604-m381-m12427-s23294-d170125/Saddle.html). I can't seem to find any part numbers to reference. I looked on OEM parts (https://www.oemmotorparts.com/oem2.asp?M=Aprilia), but they don't list the RS4. The nice thing about OEM is that they do list part numbers. Looks like you'll also be needing the tail light.


02-15-2016, 04:47 PM
Here is the list of parts rear seat cowl/cover:

1 x 85883700XN6 Saddle Cover (Black)
1 x B043934 OEM Aprilia Tail Bone Bumper Pad (RS4)
1 x 858838 OEM Saddle Cover Base
2 x AP8150450 Washer for Shafts D5
2 x AP8102504 OEM Aprilia Gasket
1 x AP8150421 OEM Aprilia Self Tapping Screw 2.9x12mm
1 x AP8144565 OEM Aprilia M4 Plastic Washer (White)
3 x AP8150413 OEM Aprilia Self Tapping Screw
2 x AP8220238 Rubber Spacer
1 x AP8152043 OEM Aprilia Hex Socket Screw M4x10
1 x 893259 Fixing Pin
1 x AP8150158 OEM Aprilia Washer 4.3x9x0.8

1 x 898595 OEM Aprilia Saddle Cover Decal (optional)

02-15-2016, 06:40 PM
Awesome! Thanks!

02-15-2016, 07:01 PM
I've had a quote from my local dealer for the parts, a little under 95. The shipping cost and import tax to the UK would make ordering this from the US more expensive than this, so I guess it will be the way to go.

First dry (and sunny!) day for a while so I've now fitted the Giannelli exhaust:


Looks good, wasn't too difficult to fit. Didn't remove any fairings, so it was a bit slow going, but still just about do-able. The most difficult part was one of the bolts at the manifold. The way that the stock exhaust curved made it almost imossible to use a wrench of any kind (and I've got about 5 different ones). Ended up using an standard allen key in the end. Thankfully the Giannelli was a much straighter exit from the cylinder.

You may also notice from this picture that these have also been removed:
Again taken off without removing the rear fairings, but taking time and being careful, job done.


Looking fine. Passenger seat doesn't look that bad, but I guess with no rear foot rests there isn't much point in having a seat. So I think the cowl will be ordered.

02-15-2016, 07:45 PM
So I obviously took it for a quick ride to see how it performs. Regardless of Giannelli's claims about being "made with several sections, manually coupled, welded and checked to ensure a good torque increase from lower rpm", well not quite. The engine has definitely lost a lot of it's low down torque, but get it into the power band and then it starts moving. There is pretty much nothing below about 7k, but it really comes alive at around 9k and goes way past 11k in the lower gears. Which is just OK on the flat but any kind of load and it's struggling a bit. However this may improve with a bit of tweaking of the carb. I've always thought that the jetting was a little out, and this has been more noticeable as the temperature has dropped, to the point of where it seems to run better with the choke on, but is obviously a bit too much at idle. A couple of times today it seemed to stutter quite badly, time to hit the technical section and seek some advice!

02-16-2016, 02:10 PM
Decided to pop in one of the main jets that I had on hand hoping that it wouldn't be too much. Probably should have bought a kit with a range instead of just the 100 and 105 but went with it.

Fitted the 100, started up fine, revs freely enough with no load. Out for a ride to see how it really responds. Wow, starting to feel what these little bikes are really capable of. Think it even hit 60mph on clock today, will have to get a GPS reading to see how close this is (probably a couple of mph generous at least). It pulled really well and was surprisingly responsive to slight changes in throttle even while in the power band. I think that even some of the lower end torque had returned with the increase in fuel provided by the new jet.

Always felt before fitting the exhaust that the 95 main wasn't actually big enough. Wish I had tried this earlier. Obviously the new exhaust has changed the power dynamics but feel that this would probably have made a significant impact with the stock pipe.

Slapping in the only jets i have is not a very scientific approach and a few more adjustments could hopefully yield even better low end response and maybe even abit more at the top!

Reed valves next?

The 17.5mm stock carb appears to be working well at this point so think I may invest in some more jets, needles and maybe a GA212 atomizer.

02-17-2016, 01:49 AM
This seems like a good place for it too. Here's the URL (http://www.ebay.com/itm/ABS-Injection-Fairing-Kit-For-Aprilia-RS4-125-and-RS4-50-2011-2014-Bodywork-NEW-/272047979206), it will probably dry up over time, but it should leave the searching possible. This entire kit is less than $500. That's a pretty nice price for being able to do a conversion. ;)


02-19-2016, 07:02 PM
Well after my last venture into fun with flammable liquids I decided to get some of these:


Pleased to report that they did exactly what it said on the tin! So much easier when you have the right tools for the job.

Things were running ok with the bigger jet I had fitted but it was only a 100 and the Jet quoted for the 'full speed' RS4 50 sold in Switzerland is a 108. So I figured that I would try the 105 main that I also had. When I got the main jets I also purchased a 34 idle jet, wasn't sure exactly what was in the stock PHVA (and I didn't check it last time when I had the carb cracked open!), but have seen some of the Derbi's run a 34.

Fitted the 105 and checked the idle, it was a 32. The bike was idling ok but thought I'd pop in the 34 to see if it would help provide a little extra at transition to the atomiser. Adjusted the mixture alittle to compensate for the larger idle and it seems to be running OK. Will need another tweak really to get it just right but is acceptable and there are no issues starting. The rpm's pick up ok, so again I dont think I'm a million miles away from where it shoud be.

After reading how imbalanced things can become very quickly with carb jetting/tuning, I think I've been pretty lucky.

Decided against fitting the malossi reeds at this time. Probably wise not to be changing too may things all at the same time anyway.

02-19-2016, 09:38 PM
Variable isolation is a very good procedural practice when trying to figure out problems. If you aren't having a problem, and you want to do upgrades it is okay to put all the bits in at the same time. It's actually a good thing if they are significant jumps in level. For instance you get a performance top end, well then you should get a performance pipe, carb, ignition, etc. Because they wont perform properly without the other proper stuff.

My experience with the Malossi reeds is that work quite well, even on mid level performance stuff. I haven't yet tried them on near stock, but I wouldn't have any hesitation putting them on a stock bike.

02-20-2016, 08:16 AM
ToraTora, I hear what you're saying. Thanks for all your advice, it's really appreciated.

Also having reservations with changing the reed block for the Malossi one as it could potentially lose me more low-mid torque than has already shifted since the installation of the giannelli exhaust. I'm sure they would be ok, I guess I could fit them and return to stock if not happy with them.

02-25-2016, 06:05 PM
Thought I would try the VL6 Malossi reed block. I got the 'Karbonit' variant wich I believe are actually a type of fibreglass instead of carbon fibre. Allegedly they are more hard wearing than the carbon. The petals that come with it are 0.3mm.

Anyway I fitted them with the stock intake manifold and I had to adjust the idle screw to keep the bike ticking over. Thought this would be ok, but it was screwed in a fair amount. A quick blip of the throttle and the pickup seemed fine. But although the revs dropped back to idle they seemed to be taking a little long to get there. Even with the idle as slow as i could make it without cutting out, the return was laboured.

Took it out for a run and the top end of the rev range was definitely increased and it felt like it pullled stronger through the powerband. But that slow return to idle was quite noticeable, not the usual engine braking that you would expect. So I posted a new thread in the technical section on this board and the general consensus was a new intake manifold would probably be the way to go. Out with the Malossi reed block and back to stock. Quick retune of the carb and was running ok again.

The stock manifold looks like this:

301343 301344 301345

Very interesting port work on the body of it too. Inside it is approx 15mm x 20mm opening out to about 20mm x 34mm at its widest point.

The intake is not in the centre of the manifold by quite some way and even the connecting tube has a slight bend in it. This sends the carb off at a slight angle out and down, and it has to, if it came out straight it would be obstructed by the part of the crankcase and the oil resevoir. It is all pretty tight in there, everything has its place.

02-25-2016, 06:48 PM
It seems that the Malossi inlet manifold is a popular choice and comes highly recommended, but I couldn't seem to find one that did not come with a reed block as well, and I already had one of these. So looking at the alternatives, the Polini appeared almost identical in design, and the other one that had caught my attention was the Stage6.

301347 301350 301351

The Polini inlet looks squarely in the centre of the manifold but the angle looks about right, the stock one is about 20 degrees I think. Part of its design allows for the inlet tube to be rotated, and in order to miss the crankcase in the same way that stock one does would need to rotated. But would this then raise the carb in such a way that it would be obstructed by the oil resevoir.

It's difficult to see from the available pictures if the Stage6 would be closer to the required orientation. It looks like the inlet is offset, but unclear as to whether it is angled. I contacted the supplier and they confirmed that it was.

It seemed like the Stage6 would just bolt on and keep the carb in a similar place to where the stock one did, so this was the one I decided to order.

02-25-2016, 09:37 PM
The rubber on the Polini, and the Malossi can rotate. This can be really helpful with the bigger carbs.

02-26-2016, 04:32 PM
Stage6 Manifold arrived and fitted along with the Malossi reed block again. I thought that maybe with potentially more flow from the new manifold the Malossi reeds might perform better.

Well there was a minor improvement but still the same slow return to idle. Just wasn't loving it, so the stock reed block is back in, again.

The Stage6 manifold is still fitted, mimics the original presentation nicely, overall a bit shorter but not enough to cause any issues connecting the air intake pipe. The carb fits into it perfectly and has a satisfying plop as it seats correctly in the recess. Little things!

The exit port measures 25mm x 30mm and pretty much matches the hole in the Mallosi stuffer.

I fitted the Malossi block with the stuffer inserted, not sure how much affect this has on things, should I have left it out? Or maybe its the 0.3 karbonit /fibreglass reeds, are they just too thick/stiff? I would have thought that there would be enough draw from the engine to operate a 0.3 reed. Maybe have to resign to the fact that it wont. Why doesn't someone do a 0.25 or 0.28 reed for the Derbi engine, this probably would be about perfect.

So reeds are parked for now.

02-26-2016, 06:16 PM

You really aren't going to see a performance boost until you do something with the top end, and pipe combination. These intake things only help when the rest of the engine is pushing for performance.

The slow return to idle is probably in the carb. I would check the idle jet/ and air/fuel mixture ratio screw. EMX and Spuds have tons written about how to set these. ;)

When you pop the top off of that engine please take a photo of the crank. I'm curious if there's a shelf in there to prevent larger ballers from easily being installed.

03-12-2016, 03:57 AM
Well it was nagging me a little that the reeds had caused an issue, most suppliers quote that 0.30mm should work ok on a 50cc cylinder. When I tried the Malossi block and reeds it felt that there were just a little too thick/stiff for the idle. But I had also seen these from Polini for the stock block:


Seemed to remember reading somewhere that the Polini reeds appeared to be a bit more flexible for the given thickness and the 3 petals similar to stock could also help this too. So ordered some to give them a try.

I had also seen and read some things about boost bottles. I know, I had also read that they are a bit of a waste of time and don't make a noticable difference etc. etc. but some bikes have them fitted as standard so there must be somethig in it.

However one video stuck out. This showed that the idle circuit appeared to running a little more efficiently and the slide had been dropped to compensate for the increase. With the bottle removed the engine just died but picked up cleanly again with it attached. So this obviously got me thinking.

So I thought it would be worth a shot, and i had a specific purpose for it and if it increased the rpm at idle a little the reeds would be worth trying again. Ordered this:

It appears that there are a number of factors to consider with these bottles to get maximum efficiency, their displacement, tube sizes and length positioning etc. Couldnt find any quoted figures for this particular bottle, short of filling it with liquid and measuring it I will just have to give it a go.

03-12-2016, 04:43 AM
Boost bottles are somewhat contentious. I only know of one bike that was fitted with one as standard. That was the Yamaha RSX100. The theory works fine (I'm not going to bother to go into it). However. It only seems to be of value at low rpm.

One thing has bothered me about add-on boost bottles. The stock one fitted to the Yamaha RSX100 had a large diameter (but short) tube from the manifold to the air chamber. The add-on bottles always seem to have a narrow, long tube connecting one to the other. I question the practical worth of the add-on bottles.

03-12-2016, 06:37 AM
I am with Spud on this one for sure.

I question the practical worth of the add-on bottles.

The Yamaha YSR came Factory with a Boost Bottle and it was more like what he described.


There are a few other Yamaha engines that also utilized a Boost Bottle but I can't recall which.
Yamaha did a lot of work and had an amazing White Paper out on the theory and application.
I (somewhere) have a hard copy of this, wish I knew where that "safe place" was I put it ...

The YSR's would run like total crap without it so we always left it in place until the engines moved to more aggressive builds (YZ125).

Basic guidelines for a Boost Bottle on a single cylinder are 60-80% of cylinder volume.
Included volume = Bottle, Tubing, Fitting
Boost Bottle should be installed between the Carb and Reed Cage and as close to the Reed Cage as possible.
Larger volume helps lower RPM, smaller volume helps mid RPM.

03-13-2016, 05:41 AM
The bottle did come with reasonable sized tubing (inside diameter approx 5.5mm). This fits in with Spudguns observations and the pictures supplied by Xero. Which then, following the fitting instructions, didnt make this a straight forward bolt on. The standard vacuum tube is approx. 4mm I.D. The Y connector supplied is a great fit for the supplied tubing but a bit of a challenge for the standard stock clear vacuum tube. This size tubing is obviously used as this fits the vacuum nipple on the Dellorto carb.

These are the supplied fitting instructions:


I didn't want to cut the stock vacuum tubing, just in case I wanted to remove the bottle and the supplied tubing was never really going to get a proper seal on the carb. I did a quick test of this and the thickness of the tube meant that it would be almost impossible to get a clip inbetween it and the carb body.

To get this to fit properly I needed to order some better sized tubing and another Y/T connector and clips.

While I was waiting for this to arrive I found some narrow walled 4mm I.D. silicone tubing. Not strong enough to use on its own but just about right to use as an inner sleeve:


This made a tight enough fit to the vaccum connection on the carb for a test at least. Wouln't have liked to run for any extended time without a clip on it, but all good to try it out and see if made a difference to the idle as I had seen in an online video.

Outcome: An increase in the idle speed of around 400rpm.

Well it's definitely doing something and quite surprised at this much increase. This is exactly what I was looking for. Hopefully enough to fit the stiffer reeds and still maintain a correct throttle response.

This is all well and good, but it still needs to be actually fitted to the bike somewhere. Most of what I had read suggested that this needs to be above the cylinder for it to operate correctly. Obviously this limits the available choices of location and there is so little room around the engine anyway. Fortunately there are support brackets fitted on the inside of the frame that are just about high enough for this to work.


A couple of cable ties and it's in probably the only viable place without any significant modifications.


Not exactly a show piece as it's pretty much hidden by the frame and fairing but if it provides some benefit to the setup then its fine by me.

03-13-2016, 06:09 AM
The tubing length is to allow for tuning, to get the proper resinating frequency. Like Xero said Yamaha did a fair amount of research on these puppies, and published it too. They can be found on almost all of the later Yamaha two strokes. Including the RD/RZ 350 ish bikes. And yes these are all about the lower/mid range RPMs, one of the places we tend to give up with going for more HP, so it's nice to have a device that will help offset some of the sacrificed territory.

BUT these resonators are not a panacea for a system that is already troubled. You should first get the bike running properly, and then add things like the resonator. Adding this on top of a system that already has issues just increases your number of variables. When tracking down a problem the key is to isolate the variables. ;)

Clint Eastwood
03-13-2016, 07:31 AM

Boost bottles were vacuum-controlled chambers that were connect after the carburetor, but before the reed valve. The suction of the fuel passing by the boost bottle orifice on the intake stroke allowed the boost bottle to suck up excess fuel when the reed valves snapped shut. This extra fuel could then be mixed with incoming fuel on the next intake cycle. It started out as an aftermarket product, but even Yamaha realized that it helped the mid range and put the Yamaha Energy Induction System (YEIS) on its production two-strokes. There is no reason why it wouldn’t still work todayhttp://motocrossactionmag.com/wp-content/uploads/2015/01/huskybooststrap.jpg

Boost bottles came in all shapes and sized. Engine tuners, like Pro Circuit’s Mitch Payton, spent a lot of time trying to find the perfect volume. This Pro Circuit Husqvarna has a special manifold for the reed block to incorporate the tube. The boost bottle is strapped under the gas tankhttp://motocrossactionmag.com/wp-content/uploads/2015/01/intankboost.jpg

The trickest boost bottle ever was on Andy Jefferson’s Pro Circuit Husqvarna. It was built into the aluminum gas tank.302834

The factory teams also used boost bottles. The rectangular Yamaha works boost bottle was hose clamped to the OW’s remote shock reservoirI see a few worthy benefits here - especially on larger setups with carbs over 17.5mm in size riding on public roads with low speed traffic or off road trail/enduro/trials riding http://www.totallyamaha.com/snowmobiles/aaTECH/BoostBottle/boost_bottle_theory.htm
The tube/bottle has a charge of pre-atomized fuel that is instantly ready when you go from off to on throttle. It assists in filling the cylinder for a fraction of a second until the carburetor venturi gets enough velocity to fully atomize the available fuel. The advantage is that it brings up the torque and horsepower sooner in the bottom of the rpm range.If weight saving wasn't so important on trials & hybrid enduro machines i guess Yamaha/Sherco would still be mounting a bottle somewhere on the chassis or molding it in with the fuel tank.

Think i'll give one of these things a go on the SX sometime' since its now semi-converted to low speed trail/deep mud surfaces feathering the clutch lever in mostly 1st/2nd/3rd gear where the revs hardly reach past about 4,000 rpm. if the pipe's too big for the Polini's vacuum connection then id have to swap the intake manifold for the Doppler ER2 which has 1 extra connection so i could connect it there http://www.maxiscoot.com/pipe-d-admission-doppler-er2-diametre-19mm-montage-rigide-derbi-cgn325877.html
ports: vacuum, oil injection, boost bottleHopefully not though' because it's the same length as the stock manifold which i think didn't leave enough space for the Polini air filter without it touching the rear shock absorber spring.

03-13-2016, 03:05 PM
The tubing and T piece arrived , so I've completed the fitting of the boost bottle. The new tubing fits well on the carb and the new T piece makes it easy to attach the fuel vacuum tube. I tried to keep the distance from the carb to the bottle as short as was possible, but this is really determined by the mount point of the bottle.


I'm not seeing quite as much of an increase in idle rpm, but the inside diameter of the new tubing is 1.5mm less than what was supplied with the bottle, so I expect this is probably a factor.

So, how does the bike run? As expexcted there is no super launching turbo style boost. But I definitely think that it feels a bit smoother coming back on the throttle and the low to mid rpm seem to be a bit more responsive, reclaiming some of the loss experienced after fitting the Giannelli exhaust. Overall, happy with this.

03-13-2016, 03:31 PM
These arrived in the post as well:


As the boost bottle allowed me to re-tune the idle and I've fitted the Stage6 intake manifold, I thought it would be worth revisiting the replacement of the stock reeds. After removing the stock reed block I noticed that one of the screw heads didn't look too great. I was still able to carefully remove it and refit it tight enough but would not want to have to do this any more than once or twice or it will end up completely rounded. Can't seem to find OEM replacement screws only a complete new block.

Well luckily these reeds work well and definitely perform better at the higher rpm's. I wasn't looking forward to having to refit the stock ones considering the state of the screw.

03-20-2016, 01:55 AM
With all of the changes made so far the engine has really come alive and is producing what feels like good power. Was it enough to pull a 14/53 gearing setup? The accelleration when it hits the powerband is impressive and to be honest, a bit crazy in 1st and 2nd gear, requiring some seriously quick shifting and clutch operation.

I had an Afam 14t front sprocket so thought it would be worth fitting and seeing how it went. There wasn't enough adjustment available on the current
chain, even when the rear wheel spindle was right up against the swingarm it was about 1 link to short. So I fiited a new Regina chain as well.


Things are definitely a bit more sedate now, it's not quite as crazed but still has very acceptable acceleration. That said, I think its on the limit now for the power that is produced by the current setup, although moving to a bigger carb than the 17.5 PHVA is an option. I still find myself 'caught between gears' at times, but this was the case with the 13t that it previously ran and to be expected on a small cylinder machine. It's more than rideable though and the extra mph for a lower rpm is welcome. The engine is probably a bit happier that this also keeps the max rpm down.

Top speed is just a fraction short of a real 60mph which I dont think is too bad for the stock 50cc cylinder.

03-20-2016, 02:37 AM
I think I'm almost at the point of needing to let things settle with regard the engine and its associated mechanics although I might look to change the carb after a little more research on the logistics of this.

A quick summary and a list of the items changed and fitted thus far:
Giannelli exhaust with Carbon Fibre silencer

Polini 0.30mm reeds in stock block

Stage6 Intake Manifold

Dellorto 17.5 PHVA (stock) but with the internals changed
110 Main Jet
34 Idle Jet
A13 needle (middle notch)
GA210 Atomizer

PM Tuning Boost Bottle

Afam 14t front sprocket (14/53) and Regina chain

Stage6 Teperature Guage with spark plug / cylinder head sensor

Passenger footpegs removed

I have ordered the rear seat cover/cowl from my local dealer so this will hopefully be fitted next week sometime.

Also on a purely cosmetic note, i fitted these:

MTEC 35w Headlight Bulb (Super White) Colour Temp: 4350K
Osram W5W (Cool Blue Intense) Colour Temp: 4200K

Not any brighter as such, but look a lot nicer!

I'll give the bike a good clean and take some photo's soon then get them posted, might even get a go pro type camera to record some footage of how it running on the road.

Future plans will include a different top end of some kind, possibly a crankshaft depending on which top end I get, a different carb, clutch or just springs and maybe some shorter clutch/brake levers?

04-07-2016, 06:54 PM
Here are a few pictures, they're not great, taken on my phone which is definitely not the best!


Here is some footage of it on the road.


04-07-2016, 07:11 PM
The rear seat cover/cowl also arrived and has been fitted.


Looks good.

04-07-2016, 09:08 PM
Nice vid. The engine sounds pretty good! :)

04-08-2016, 06:06 PM
Cheers ToraTora.

It feels like its running ok and the plug looks a decent colour. Have rejetted the carb since the list I posted earlier in the thread. Dropped the idle back to 32 but increased the main jet to 112. For a stock 50cc it goes quite well.

04-08-2016, 07:59 PM
That tail section does look good!

Sweet bike. :)

04-08-2016, 08:19 PM
I need this tail section for my bike! Looking good mate

01-14-2017, 03:29 PM
Dellorto 17.5 PHVA (stock) but with the internals changed
110 Main Jet
34 Idle Jet
A13 needle (middle notch)
GA210 Atomizer

Was reading your setup. Did you think about "free flow area"? Atomizer/Needle combo not restricting flow from main jet.


The free area of a 110 mainjet is 0,95 mm2.

However using a GA210 with A13_n3 only have a free flow area at WOT of 0,89 mm2

Using this chart, you would notice that at least a GA212 atomizer could be required - or a thinner needle (A20)


01-15-2017, 09:09 AM
I hadn't but this is very interesting. The needle mapper that emX50 produced is an amazing piece of work, you seem to have got to grips with it very well too.

I ended up running a 116 main jet in the end which would have affected the calculations even more I guess. It definitely wasn't perfect but ran well and the plug colour was good. The atomiser and needle were the ones that the dealership put in when de-restricting it. They fitted a 95 jet at the same time so this probably would have been more of a match to the free flow area?

Now I have fitted a PHBG carburetor and it needs a little more work to dial it in, but again the bike is running ok, I'm planning to update this thread soon with this recent change soon (fitted this week!).

Would be great to have a needle mapper for the PHBG.

03-17-2017, 02:08 PM
What's your current setup?

03-17-2017, 02:29 PM
@hdvp with the new PHBG 19mm carb I'm running a 96 main with a 45 idle. Like you I found that the original 50 idle was too big and the mixture screw was all the way in and still too rich. Have also tried larger main jets but the 96 is giving me the best performance. Still using the W7 needle.

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04-15-2017, 03:33 PM
Do you recall how fast bike was running stock derestricted from dealer, when you were running stock exhaust and carb?

I simply can't figure out how the Swiss version are able to pull a 14/53 gearing up to 85-90 km/h....

04-28-2017, 01:07 AM
Stock deristricted from the dealer was a bit over 50 mph. This was with 13/53 gearing. Carb internals were also different to the quoted Swiss ones though.

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