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Thread: Starting problem, dealing with compression? Lots of technical info included

  1. #1
    apriliaforum prov-nov
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    Question Starting problem, dealing with compression? Lots of technical info included

    First off, if you don't like to rear, I tend to make a novel of what I'm dealing with. Everything is in the details and I'm just trying to list everything out at first to show what I'm dealing with.


    So lately I've been having a problem trying to get my bike to start. Well, ongoing issue more so.

    Last time I took my bike for a ride was early/mid March.

    I tried starting it up two weeks later and nothing, no matter what. Found my carb kept overflowing so I think I've got that addressed since readjust my float level (Mikuni TM24).

    Since it wasn't starting with the carb, I pulled that off the intake and sprayed some starter fluid through the plug hole, put the brand new plug back in and cranked it over and nothing.

    I pulled the spark plug and put it up next to the frame and found that yes I do have spark. I never put it next to the cylinder head because the frame was right there. I then found out that the ground was mislocated. It had been attached to the bolts that hold down the reed cage. Since correcting this issue, I have not tried to start the bike. Instead I started to rule out other issues just in case.

    Compression: Upon first crank, I get about 60psi and the 2nd or 3rd pump brings it up to 90-92 psi. The only thing I found for what the compression should be is that around 100psi is ok. Now given that my bike has a big bore kit to it, I don't know what the psi should be but after talking to a local scooter shop, they said 120 is low and should be around 140psi.

    With that information, I decided to pull the top end off and check for ring wear. I then removed the head, piston and the single ring for inspection. Checked the ring gap and it came to 0.013" (0.33mm for you metric folk). From what I can find, new should be between 0.15-0.30mm with a wear at 1.0mm. So according to specs like those, the ring should be good, right?

    Piston size: It's a 49mm (measured size of 48.8mm). So apparently I have a 75cc kit.

    After looking at some bore kits, they specify that a longer crank is needed to fully utilize the bore size. I have no idea if mine came with an aftermarket crank or not.

    This is such an odd starting problem that's got me confused. I'm about ready to pull the trigger on a new top end, possibly a Polini 80cc kit just so I know that everything on the top end is new. It would eliminate any confusion(a given for good compression) and narrow it down to the carb since I can verify that I am getting spark.

    I have also looked into getting a 50mm Malossi piston and having the head bored out more but due to the tight fit from the cap to the head, I'm uncertain if that would even be a possibility.

    Any advice? Recommendations on bigger bore kits in the 70-80cc range?

    I'm just using this bike for putting around town as my main toy is Ducati 916.

    The top end:


    The bore, no scratches or marks of any kind:


    The piston:

  2. #2
    apriliaforum prov-nov
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    Oh ya.. last I verified the reeds did have a pretty secure seal when fully closed.

  3. #3
    apriliaforum Junkie nicketynoo's Avatar
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    Did your plug come out wet ? looking at your barrel there seems to be some wear marks but its gotta be pretty knackered not to get it to fire! have you tried bump it off ? I've seen barrels on here that have have circlips gouge the barrels and they've still run but just won't tick over very well. If I were you I'd build it back up and look elsewhere. Smear the bore with 2 stroke as you build it up and check the compression again.,but make sure your battery is fully charged so its cranking over well. It would be worth having a quick check on your flywheel too, make sure the key holding it in position is ok. if its slipped you'd still get a spark but the timing will be out ,but your plug would come out wet then.
    1999 rs50
    standard 50cc barrel+piston
    12mm phbn carb 86 main jet
    standard airbox
    conti racing clutch+uprated springs
    exhaust- Gianelli-carbon end can
    reeds- not sure
    bank account-empty

  4. #4
    apriliaforum Junkie camb00's Avatar
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    Check crank oil seals, And what size reeds have you got in there now?

  5. #5
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    I honestly can't remember it coming out wet. Then again, I didn't immediately pull the plug after trying to start it. Before ripping everything apart, I did try bump starting it in case of a weak starter and that didn't work for me.

    The barrel may look like it has some wear, but that would just be normal, It still has a nice cross hatch pattern from honing it as they come normally. There are no gouges or scrapes of any kind from rings.

    Good idea on throwing some oil in the bore for reassembly. That should give a good seal until it's turned over again. Maybe I'll put more than quick squirt of starter fluid in it this time.

    Never thought about the The timing key, seems like a simple check.... as long as the nuts holding the rotor aren't too much of a pain to get off.

    As for what size reeds I have, as far as I know they're stock, something I will be replacing this summer as I ride it more.

  6. #6
    apriliaforum expert chrism15's Avatar
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    with 75cc your gonna need more than stock reeds really, id say more like 0.30mm

    as said above (camb00) check crank seals, they can leak at any time
    ----------------------------------------------------------------------------------------------------------------------------------------
    check for where on piston and barrel:

    get a couple of peices of newspaper on a flat surface

    place the barrel upside down so that the top of the cylinder will be sitting on the newspaper making a seal

    remove piston ring!

    make sure the barrel is wiped slightly so no dirt or gunk ect is going to effect it

    drop the piston into the bottom of the cylinder (but it will be at the top since its upside down obviously)

    the piston SHOULD slow down at the exhaust port and gently come to a stop without a thud

    if it drops straight through without slowing down past the exhaust port and the makes a light thud then its worn

    ------------------------------------------------------------------------------------------------------------------------------

    if you go for a 80cc polini bore kit then you seriously need to check if it is a race crank or not, a stock crank will not last long with it otherwise and will probly snap. if you got the stock crank nw then you should look into getting a race spec crank - doppler ER1?

    make sure spark plug is clean, wire brush the electrode lightly if need be

    considering you havnt tryed starting it since you adjusted the carb then i think you should- problem might be gone and it may start and then we are trying to diagnose nothing, if it still doesnt start then we can rule that out but if it does then ride it and report back with any other symtoms
    Engine Always Screaming!

    Its Not What You Ride Its How You Ride It

  7. #7
    apriliaforum Junkie camb00's Avatar
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    As for what size reeds I have, as far as I know they're stock, something I will be replacing this summer as I ride it more.

    Well theres your problem, You reeds are probably too thin and are floating. get some carbon reeds about ~ 30-35mm thick

  8. #8
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    Just flipped the cylinder head upside down to check the seal on it. With the rubber o-ring around the top I was able to for certain get a good seal. I lightly dropped it in through the bottom and it did slow down as it approached the bottom. I could visually see it suddenly slow down as it reached the bottom and gently tapped the table.

    I measured the bore size of: 1.929" (48.996mm) I did multiple measurements to ensure repeatably.
    Piston diameter just below the ring: 1.9215" (48.806mm)

    The bore size isn't elongated either as I measured perpendicular from the initial measurement and got a variance negligible to measurement error. So I don't foresee that being a problem.

    Thanks for the information about the reeds as I haven't figured out what size would be best for my application. I'll have to order those up soon.

    Tomorrow I should be able to have it all back together and see about starting it up and what happens. To see if it at least fires without having to put fuel through the carb(that's just messy to deal with at times), I may just leave it off and spray starter fluid through the inlet or spark plug hole. If I can get it to fire, then I'll reattach the carb and fuel it up.

    Oh ya, spark plug is a brand new one. I made sure I started with that to possibly eliminate it as a problem. The gap on it, from what I could find, is between .6-.7mm.

    Part of me doubts that I have a race crank so I think I'll add that to the "to-do" list. Working on two bikes kind of drains the funds, especially when one is a Ducati. Until I can get the crank, I think I'll just be taking it slow on this bike as I'll now have that little voice in the back of my head telling me if I push it too hard I risk snapping the crank. Plus this bike only sees "spirited" riding rarely. I mainly use it for transportation to and from work during the summer, then again, anything can happen at any time.

  9. #9
    apriliaforum expert chrism15's Avatar
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    75cc would probly snap the stock crank quiete easily at high rpm, patrick has a few pics of i think 2 stock cranks that are snapped

    get the reeds done early coz that will cost u a bit of compression, and if the reeds are to thin theres a chance they cud rip off and then wizz round ur crankcase ruining your crank bearings
    Engine Always Screaming!

    Its Not What You Ride Its How You Ride It

  10. #10
    apriliaforum Junkie camb00's Avatar
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    It's not all-ways the High rpm's that snap a crank. Low rpms can be just as bad because of the heavy fly wheel the momentum of it can cause shearing.

    But lets try and get it running first haha

    Here is a good crank: Crank

    They ship to the US and its costs €34,- so in total that's €143,50 which is about $183 Not sure if thats good value for you but yeah.

  11. #11
    Tecnologia Reparto Corse RS_Patrick's Avatar
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    Single pistonring rings have way stricter tolerances than normal, and 1mm gap is insain anyway, stick to 0.50mm max and your ok.
    Wen mesuring the gap make sure the ring is about 5mm above the axhaust port, and push it sraight by using the top of the piston and than mesure the gap.

    Think you just want to have the bore lightle honed to get the little ledge out on top of the barrel so the rings dont hit that anymore, than test if the piston is not worn, or the cilinder diameter is worn out to much as followes:
    Put a newspaper/magazine on a table, put the barrel inverted on that, than take the piston with out rings and drop it in there the way it normaly would go.
    It should than drop and slow down almost to a halt wen it passes the exhaust port and gently come down on the newspaper with out a tud.
    If it does that than the piston is still the correct size and you just need a new ring, if it falls trough than you also need a new piston, or a whole new cilinder kit.
    keep in mind you cant hone or bore aluminium kits much as they got a thin Nikasil layer coating on the inside of the barrel.

    Wen your mesuring the compresion use the electrict starter so the engine spins round fast and than look at what compresion has been mesured.
    Should be 110~140 PSI

  12. #12
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    So I just got done reassembling everything and checked the compression again. This time it maxed out at 70psi. I'm starting to think as camb00 said it could be the crank seals. It first hit about 50-60psi and after a few seconds it maxed out at 70psi.

    I reassembled everything as per the manual for properly aligning the piston in the barrel for the ring orientation and lubed up the barrel with the 2 stroke oil to ensure a proper seal. Added more starting fluid this time to see about turning it over and nothing.

    Guess it's time to replace them along with a new crank/ bearings.

    On the vsmotoparts website for the crank recomended, the Doppler, that it comes with a larger bearing for one side as it does on the adrenalin-pedstop site, but it's also almost €8 cheaper.
    http://www.adrenalin-pedstop.co.uk/p...ines-2016.aspx

    Is it safe to assume that it would come with it though not mentioned?

    I'm thinking for ordering parts dealing with the crank, this will be included or replaced all at once:
    Crankshaft
    Required bearings
    Crank seals
    Woodruff key for generator/ spark timing
    .30mm reeds

    I think that about covers the crank area for replacing everything critical. I added the key and reeds in there just to replace it for good measure, just in case.


    On the topic of heads, I researched some older threads and found that the Top racing Pink 80cc cylinder head would be a good one for reliability.
    http://www.apriliaforum.com/forums/a.../t-228857.html

    Seeing as it has a cast iron head, that would wear much easier than an aluminum head, but potentially increased ring wear. I'm not really concerned with high power numbers, but still want a bigger bore so it seems that might be a good choice. It was either that or this: http://www.adrenalin-pedstop.co.uk/p...ines-1774.aspx

    Any feedback on the heads? That may be my next purchase if fixing the crank doesn't completely solve everything and I feel the need to replace more items.

  13. #13
    Tecnologia Reparto Corse RS_Patrick's Avatar
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    Head is to top cover of the cilinder, your talking about the barrel, and cast iron barrel like the Top Performances dont wear much faster than Alumium kits, the advantage with Cast iron is if something happens you can bore it up, with aluminium cilinder you got a chrome/nikasil coating on the inside of the barel wich is verry thin so a scratch in there thats a bit deep is the end of it.

    If i where you i have a really close look at the state of the barel and see if your piston still fits it correctly by doing the test i described, if not you need a replacment piston, and you probably have to make sure its the same type/brand of piston since its not a stock port layout barrel.

    You linked to adrenalin pedstop for the crank and it says there that it comes with the left hand side bearing, and all these cranks should have them packadged with them from the factory.

    You also need an engine gasket set, and i would replace all engine bearings with C3 ones, and replace all seals for fresh ones so you dont have to worry about it, pay attencion to the waterpump, as they dont like being run on water at all and fail from that, luckly replacment kit is really cheap.

    But a Malossi Reeds set, than you get 0.30m, 0.35mm and 0.40mm for just a little more than just one set of reeds and you can be sure you got the right thickness of reeds.

  14. #14
    apriliaforum prov-nov
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    Can you post a link to the Malossi reeds set? I think I found some but unsure if it'll fit my motor. That and I'd hate to order the wrong one.

    I did do the test as you described, see one of my posts prior to you initially posting, and did notice the piston slow down quickly right before it tapped the table. It did not fall straight through but for surely slowed down around the exhaust port.

    I'm thinking that I might just buy the Top Pink 80cc kit. Partially because by replacing it, it's one less thing that might be the problem. Plus getting an exact replacement piston would be a pain it seems. I can't find anything under the KRD brand name.

    Thanks of the idea about replacing the water pump, it is cheap to replace it seems. Slowly this will be getting repaired now it seems.

  15. #15
    Tecnologia Reparto Corse RS_Patrick's Avatar
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    Official Malossi USA store: http://www.malossistore.us/Prodotto....0&C=2712430.C0

    Pink TopPerformances might be a problem since your in the USA, they dont ship any more to the USA as far as i know.

    The whole Kundo/KDR site doesnt do a thing, been that way for a while now, no clue why.

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