View Full Version : A few AM6 engine questions.

06-07-2014, 01:55 PM
With my RS50 running sweet as a nut (as per the spec in my signature), I've now started to build the engine that should've gone into the frame to start with. It was my original intention to build a 'bulletproof' engine that could withstand further upgrades and still be completely reliable. Although I've learned much since I've been with this Forum. There are a few gaps in my knowledge. So without further ado.

The upgraded engine has now been fitted with a Doppler Endurance crank. I have a set of uprated clutch springs. Will a set of standard clutch plates endure any top end that will fit unmodified crankcases?

The engine in the bike will be returned to its original specification before its removed and replaced with the upgraded engine. This means; An original factory 50cc top end will be put on. The SHA 14/12 (but up jetted to suit the Arrow exhaust I have on the bike) carburettor will replace the 19mm currently fitted. The factory reed assemble will replace the Polini reeds. The 13T gearbox sprocket will be replaced by a 12T original. In summary, we're looking at a standard derestricted bike here. My question is:

What sort of performance could I expect from an RS50 that completely standard, except from an Arrow 'street' exhaust and an upjetted SHA14/12 carburettor?

06-07-2014, 06:42 PM
They go horrible with standard carb specially the sha carb my Rx wouldn't even pull a 11 tooth with a tecnigas exhaust with a sha carb and I only just did 45mph

06-08-2014, 09:48 AM
Stock clutch plates are fine for most street setups... the SHA 14.12 Carburator is not though, but still should get you 50mph with 12/47 gearing and an Arrow pipe.

Even with a Big Bore kit and the SHA 14.12 carb your not gonna get much further as its ment as a restriction for the bike, you get more performance out of the 19mm carb by it self with the Arrow pipe, than with the SHA 14.12 carb with a Big Bore and the Arrow pipe.

You can get Kevlar Clutch plates, but i only would get those wen the old plates are worn out.

06-08-2014, 12:50 PM
Thanks for the information. It's just what I needed to know. I intend to sell the RS50 next year with the 50cc engine (with the upjetted SHA carb) fitted. The upgraded 70cc engine (complete with 19mm carb) I'll sell separately, as I'll get more money for it out of the bike than in it. I just needed a base idea of the performance level of the bike with the 50cc engine.

06-08-2014, 01:48 PM
The upgraded engine has now been fitted with a Doppler Endurance crank. I have a set of uprated clutch springs. Will a set of standard clutch plates endure any top end that will fit unmodified crankcases?

I have used the Barnett High performance clutch kit on numerous AM6 bikes, 50cc, 70cc, etc, and it has worked very well. It comes with springs and discs, and works fine with the oem pressure plate / basket assembly.

What sort of performance could I expect from an RS50 that completely standard, except from an Arrow 'street' exhaust and an upjetted SHA14/12 carburettor?

My first RS50, which was a 2000 USA Spec RS50 was bone freaking stock! It also had the SHA 14/12 carburetor you're describing. Stock exhaust, stock everything! I think I geared it to be 13:47 and It went 60miles per hour in a full tuck at like, 12000 RPM or so? If I didn't get a full tuck, it would immediately slow down. It really liked to cruise at 45mph though. I would say with the Arrow street exhaust wouldn't change the power that much, maybe shift it a little bit higher so it will get to 60 a little bit easier but the midrange would suffer and you wouldn't be able to short shift it as easily, but I don't have any experience with it.


06-08-2014, 02:18 PM
Thanks for that. I've only had the bike Legal on the road for a week, so I'm still learning how to ride it. I'm amazed that the engine (in its current configuration) can withstand the punishment I'm giving it. It was my original intention to build the bike that an average 16'er couldn't destroy. This meant mating a mild 70cc top end with a bombproof bottom end. The reducto being: Mr spotty teenager could fit anything to the engine and it wouldn't explode. Thus far, I've managed to get an indicated 60mph out of it (and it was still accelerating). The 'happy zone' is between 45 and 55MPH in top gear. I've fluffed gear changes as my other bike is an Enfield (You press down to change up and up to change down). The AM6 engine took everything in its stride. I'm confident that that the AM6 engine can accept mild tuning while the bottom end remains stock.

However, I took on this project to make a few bob. At this time, I've made a loss. The only way I can redress the balance is to sell the upgraded engine separately. I feel that an upgraded, running engine is about 50% of the value of a stock, running 1999 RS50.

06-09-2014, 02:38 AM
Just keep it and have fun with it Barry, more 2 strokes the better. Preserve it from the average 16 year old numpty.

06-09-2014, 06:14 AM
Keep in mind that the USA version is difrent from the UK or EU version.

I would keep the bike, fully upgraded there really nice to drive.

Don think your gonna get much more for the upgraded engine than for the stock engine wen you sell it seperate, might even make it harder to sell.

06-09-2014, 10:55 AM
It's a bit of a double edged sword really. I still intend to sell the bike with the stock engine. However, the option is open to buy another wrecked RS50 and put the upgraded engine in that.

Yes, I've had a heap of fun riding the RS. Why is it that 50mph seems faster on the RS than it does on my Enfield?

06-10-2014, 11:35 AM
Because the bike is way lighter and probably has a stiffer suspension, and your leaning way more forward.

06-18-2014, 03:16 PM
Hi, i dont really know where to post this as im new (tell me how?!)

Does anyone know where/ or if i can get a side slung exhaust for the newer shape 2006 onwards rs, i am wanting one because the the design of the original expansion chamber is awful!! And i would also prefer this setup.
If these exhaust dont exist does anyone know if i could fit an older shaped rs exhaust to mine?

Many Thanks

06-18-2014, 04:12 PM
Should have posted in here by clicking the "Post New Thread" button on the left top of the page.

Old shape RS50 pipe wont fit, but there are under/side-slung race pipes available for those bikes, i Think Yasuni Carerra pipes for Derbi would fit as there a bit universal fitment, there also really fast, check the site to see wich of them is most suited for your setup www.yasuni.com

07-06-2014, 05:42 AM
Minor update time.

I've finally decided to keep the modified engine in the bike and sell the standard engine separately. Thanks for everyones input on this. Now, back to the plot.

The bottom end on the upgraded engine is (more or less) completed. Just a water pump gasket is needed. Now, in the engine that's in the bike is a set of Polini reed petals. I've no idea of their age. I've no idea how thick they are (They were in the engine that came with the bike). All things considered, I think they're worth replacing with a set of Malossi petals from Racing Planet. The Malossi set comes in three different thicknesses: .3, .35 and .4mm. Which ones should I fit?

07-06-2014, 01:05 PM
I have fitted 0.3 with my set-up (identical to yours) and the bike felt much more responsive low down.

07-07-2014, 11:06 AM
I'll give the .3's a go.

07-11-2014, 07:45 PM
The 'happy zone' is between 45 and 55MPH in top gear.

Very interested to hear your thoughts on the 'happy zone'.
For our bike, with the old barrel (to be re-instated next week), the happy zone was 45 to 50mph, or 70+ to 80kph.
While I could get it to above 90kph, it was clear to me that it was being pushed.

We'll have the full sized exhaust port next week, so we'll see if that 'makes it feel happier' at higher speeds.
However; this is an engine with an oval cylinder, and combustion blowpast....... plus a stock exhaust crammed full with a catalytic converter.

When you look at your spec and an additional 20cc or 40% more power (over a 50cc)...... I really think that your setup surely must have much more to give.
I do understand that every mph extra, takes much more power, but maybe your jetting is not providing the fuel mix that it really needs (to take advantage of all that kit, and capacity).

Having taken our bike from 70kph to 90+kph, I've seen the happy zone rise, with each improvement in the jetting.

Either that, or the Sticky Parts cylinder is an automatic 'avoid' for future purchases.
40% more combustion capacity should see you riding pretty, all day long at 60mph, regardless of the arrow street exhaust.

My honest opinion is that you could not do worse, than investing in the jet packs, and needle pack @ around forty five quid.

Changing the reed valves is fine; if you've got the option of re-jetting - particularly if you've already jetted to the maximum fuel burn efficiency.

BTW.... my first 250 was a Royal Enfield Crusader, with a cast aluminium headlamp dash cowling.
It was a very fast bike for the time :)

07-12-2014, 02:33 AM
Well, there's a couple of things that you've said that I can't really agree with, but I'll try to make things a little more clear.

When I laid out the specification for the engine. I wanted something that was faster than stock, but it had to be utterly reliable and would last for ages. This meant going for a mild street tune. The target speed was 60mph. I went for the Stickyparts 7 port 70cc kit as it was made of iron (Durability), had a 2 ring piston and didn't have aggressive port timings. Another advantage is that since the Stickyparts kit produced lower power, it had more bottom end torque. As the expression goes: Lower power equals longer life. The bottom end torque was another advantage (Just what you need for urban riding). This was matched up to an Arrow street pipe. These two components compliment each other well. Lastly, I went for a 19mm carburettor instead of a 21 to make sure the engine lasted a bit longer before it needed a rebuild.

So. How did the specification on paper match up to the reality of riding the bike? Perfectly! I'm still utterly gobsmacked that I got it right first time. It's the first time its ever happened! However, I'm not quiet there yet. The engine in the bike isn't to the mechanical specification I had laid out for it. That engine is still being built and will be installed next month. Let's move onto the 'Happy zone'.

I said that the engine felt 'happiest' between 45 and 55mph. This equates to an RPM between 8000 and 10000 in top gear. below 8000rpm the engine vibrates. It's like holding a vibrating mobile phone in each hand. Once the engine hits 8000rpm, it completely smooths out. The power also really kicks in at 8000rpm (Although there is a noticeable increase at 6000rpm) and keeps going beyond 10000rpm. However the engine doesn't sound all that keen to stay beyond 10000rpm for all that long. I should mention that I've yet to red line the engine. This is because the current engine still has a stock crank in it. Since I've not really pushed the engine to its limits, I don't know what the top speed is. I have exceeded the 60mph target (indicated) many times and the bike was still accelerating. But until the uprated engine is installed, I'm not going to push my luck. The carburation is pretty much spot-on. If anything, the engine is running slightly rich. The needle on the temperature happily sits half way in the first white segment. The only time is rises to the half way (that gap between the two white segments) is when I'm going up hills (Lower gears, lower speed, higher RPM). Once on the flat, it drops again. Let's move onto the uprated engine.

My original specification called for an upgraded bottom end. Due to a former owner who was probably a Gorilla, the original engine was scrap (He'd taken an angle grinder to it, to remove a stuck bolt and slotted the crankcases). So I bought in a stock engine because I had a schedule to keep. This engine wad the 70cc top end fitted and it went in the bike. I then bought a replacement set of crankcases and upgraded that engine. That engine now has a Doppler Endurance crank fitted. It also has an upgraded clutch. At Dan's suggestion, I've gone up one tooth (13T to 14T) on the gearbox sprocket. I've also bought a set of Malossi reed petals to replace the Polini items that are sat in the current engine. All of this means that the carburation will need to be rechecked (I've already got the jets to do this with).

Once the new engine is installed, I can give it a good full on thrashing. Confident in the knowledge that its not going to destroy itself. I am expecting the 'Happy zone' to shift a little. To where, I don't know. I'll check back at the end of next month.

07-12-2014, 05:48 PM
It's a good explanation.
So, as I understand it...... the taller gearing that you mention, is not fitted to your current motor.
Hence why you are doing 55mph @ 10k (the upper end of the sweet power band).

It would be interesting to see if the new gearing gets you to 60mph @ 10k - what would be a nice cruising speed, particularly as you say that it has power 'in hand'.
Are you thinking of dropping the new pinion onto your current setup, as a test?

My target cruising speed was 58mph or 90+ kph, to stay ahead of the speed restricted lorries (@90kph), so it was a very specific requirement.
I don't know whether that will be possible to achieve without an overbore (it's looking unlikely).

The original cylinder has plenty of potential for overboring, but I'm first gonna see the results of giving it a full size exhaust outlet port.
I've been told that a guy in the next village has got a milling machine, and a lathe :) either would do..... hopefully for a few quid he'll let me use one of them.

The vibration you mention, up to 8k, does not tally with our engine, which has no specialist crank etc.
The AM6 counterbalance seems to be working perfectly as the design engineers intended - throughout the rev range - really impressive.

Assuming that you got the counterbalance timing correct (as per the markings)...... this can mean only two things (discounting bearings, and incorrect assembly).

a) The crank was originally incorrectly balanced at the factory
b) The crank became incorrectly balanced, due to the additional weight of the 70cc piston

It's a cautionary tale that highlights why tuners weigh their new pistons, and try to get the weights to match (through metal removal).

Surely the new crank will have this problem sorted, as it's likely to be matched to a bigger piston....... let's hope so.

At any rate, it has reminded me of the potential pitfalls, when seriously upping piston size.

07-13-2014, 01:50 AM
Yeah, the engine in the bike has a 13T gearbox sprocket. I'm not going to bother to fit the 14T sprocket as a test, as I'll be fitting the upgraded engine in five weeks time. It just not worth the effort to change things now. On the whole, I'm very happy with the engine that's in the bike. I'm amazed that it can take the 'thrashing' that it gets. Fingers crossed, the new engine will be even better.

I'm not surprised that the engine vibrates. The bottom end is completely stock. I did give it an initial check before I installed the 70cc top end and everything was as it should be (Including the balance shaft). I'm of the opinion that the balance has been upset because of the larger piston. Engine balance is a huge subject. I once had a book called Tuning for Speed. Engine balance took up a whole chapter (This book came out before the days of balance shafts). There were two major considerations when balancing a crank. One was the piston weight. The other was the RPM of the engine at cruising speed. Counterbalancing the piston weight is obvious. Balancing the force on the piston on the ignition stroke is a bit of a dark art. My Enfield (A 500cc single) vibrates. If I put it on its centre stand with the engine at idle, the whole bike flexes. However, at cruising speed (55-60mph) the engine is completely smooth. So engine RPM is certainly a consideration. This chapter in the book dealt with other issues (Angle of cylinder lean. Sympathetic vibration, etc ), but that's not important here. I suspect the the Doppler crank with shift the 'smooth spot' somewhere else in the RPM range. It would be a bonus if the vibration was completely eliminated.

I'm not keen on messing about with port timings and over boring the cylinders on these bikes. My reasoning is simple. There's an extensive range of cylinder kits available, so why bother? If it was an old obsolete bike where tuning parts weren't readily available, then yes. Mind you. If you've got an old scrap cylinder and just want to experiment, then go for it. The original cylinder certainly has scope for tuning, but I wouldn't do it myself.

07-13-2014, 06:26 PM
'Tuning for speed'.......:happy: :happy: :happy:

The first tuning book that I ever bought :)
I, don't know where it is now........ probably in my parents loft LOL.

I remember the chapter you describe; and when I made my post....... it was in reference to that learning (information that clearly stands to this day)

Very nice.


PS. Never owned an Enfield 500, but did have a Matchless 500 single at one time.