Symptoms. starts fine, idles fine (maybe bit lean with basic setup), low to mid revs fine. But on high revs it misfires, slow topspeed and generally bad runnign (generally mixture aint igniting on every stroke... hope you understand. Tried new base gasket, carb gaskets, different carb,plug, coil+wire+cap and better (not new) ring. carb bubble is full of fuel and fatting or leaning H aint curing or even improving symptoms. Plug seems little lean but with lots of WOT driving because testing. It has got maybe 5 litres old 29cc cyl but I saw coating worn around exhaust so it hasnt got much life left, but could bad coating do that kind of symptoms. I am almost ready to buy new mill but since ddm DOTD had that sad 29cc on low price maybe I try that before ordering new.
engine problem
- Thread starter q-man
- Start date
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ok, what topend do you have?
you say it idles fine, no ups or downs? it stays nice and smooth, but in the upper rpms, it sounds like its misfireing or back fireing?
if you have the CY topend with that pretty top head piece, you may have a few things happening. mainly the head/cylinder gasket is leaking (warped head) once it gets hot. OR you may have a Crank oil seals and or beairng/s going south, so at the upper rpm, the higher case pressure is pushing thru the crank bearings and past the oil seals. this causes a popping sound like a miss fire. take your clutch carrier and bell out, grab the clutch shoes and see if you have any movement either up or down, or side to side.
if you do, you need to replace the crank bearings and oil seals.
you say it idles fine, no ups or downs? it stays nice and smooth, but in the upper rpms, it sounds like its misfireing or back fireing?
if you have the CY topend with that pretty top head piece, you may have a few things happening. mainly the head/cylinder gasket is leaking (warped head) once it gets hot. OR you may have a Crank oil seals and or beairng/s going south, so at the upper rpm, the higher case pressure is pushing thru the crank bearings and past the oil seals. this causes a popping sound like a miss fire. take your clutch carrier and bell out, grab the clutch shoes and see if you have any movement either up or down, or side to side.
if you do, you need to replace the crank bearings and oil seals.
- Thread starter
- #3
Topend is basic 29cc 36mm cy. It sounds like lack of fuel but I am not sure anymore. Oilseals might be problem but there isnt any clearence on bearings. It might be related to heating because letting engine cool 5 mins gives 1 or 2 WOTs maybe bit better than before. Whatever is causing problem i have to say that 29cc cy head has bad bad very bad coating when its almost done after 5 liters. And i bought another one.:w00t:
atom3624
Well-Known Member
Probably never a problem, but as it's at max revs., how's the tubing (any kinks in the fuel line?) and the air filter (need a clean?) before ripping it apart.
Al.
Al.
My LEO went like that at one time,I had a pinhole in the fuel line.
J.
J.
- Thread starter
- #6
New fuel lines didnt help but problem is now solved. Took motor from my onroad and put new 29cc cy cylinder and voila!! problem solved. Actually clutch,cyl and flywheel were pretty much done so I bought new 27cc cy to onroad. I might need bit taller gearing or even 2 speed to match revs to ported 23cc.
ok, ive had that topend, and overall its pretty good. the cy 36mm topends are pretty good, but the 35mm ones are poop, all of them.
you probably have a bit of heat soaking going on. earlier, did it seem to lose topend power (before it was "missing")? the more you ran, the more the topend suffered? if yes, you were seeing heat soaking, which is a condition of to high of a compression, which can be one of the main quirks with this topend in stock form. they are around 17:1 maybe 17.5:1 compression which is high. You can try High Octane fuel (100 and up) which may help. but you ll likely need to raise the cylinder a bit. i would use a cooper .025 cylinder gasket. the .005 extra clearance will lower compression a smidge, and shoudl help eliminate the heat soaking.
but you have a 2ndary condition of the heat soaking (which is running hot basically). the heat has probaly weakened the oil seals as you noted. and likely hurt the bearings in the process. this is only a theroy i have, but i think it plays out pretty close to the actaul sequence of events.
the motor gets hot.
the heat plays hell on the oil seals and bearings.
over time, the seals weaking.
part of the motor process of high rpm, is high case pressure.
the higher case pressure, pushing agains the now weakened oil seal, blows by the oil seal.
the sudden release in case pressure at high rpm, caused the motor to miss.
often you can hear a loud "Popping" noise when this happens. i suspect this is the oil seals slapping back against the crank after the case pressure (large burst of high pressure heated gasses) push past the weakened seals.
just a theroy as i noted, but i think its pretty sound.
fix is new case bearings, and oil seals. install a .025 copper cylinder gasket. when you take the cylinder off, check over the cylinder walls very well. see that its still good.
i would also replace the wrist pin bearing as well as the ring while your there. and ofcourse the other gaskets, ie, case, carb, exhaust ect...
you probably have a bit of heat soaking going on. earlier, did it seem to lose topend power (before it was "missing")? the more you ran, the more the topend suffered? if yes, you were seeing heat soaking, which is a condition of to high of a compression, which can be one of the main quirks with this topend in stock form. they are around 17:1 maybe 17.5:1 compression which is high. You can try High Octane fuel (100 and up) which may help. but you ll likely need to raise the cylinder a bit. i would use a cooper .025 cylinder gasket. the .005 extra clearance will lower compression a smidge, and shoudl help eliminate the heat soaking.
but you have a 2ndary condition of the heat soaking (which is running hot basically). the heat has probaly weakened the oil seals as you noted. and likely hurt the bearings in the process. this is only a theroy i have, but i think it plays out pretty close to the actaul sequence of events.
the motor gets hot.
the heat plays hell on the oil seals and bearings.
over time, the seals weaking.
part of the motor process of high rpm, is high case pressure.
the higher case pressure, pushing agains the now weakened oil seal, blows by the oil seal.
the sudden release in case pressure at high rpm, caused the motor to miss.
often you can hear a loud "Popping" noise when this happens. i suspect this is the oil seals slapping back against the crank after the case pressure (large burst of high pressure heated gasses) push past the weakened seals.
just a theroy as i noted, but i think its pretty sound.
fix is new case bearings, and oil seals. install a .025 copper cylinder gasket. when you take the cylinder off, check over the cylinder walls very well. see that its still good.
i would also replace the wrist pin bearing as well as the ring while your there. and ofcourse the other gaskets, ie, case, carb, exhaust ect...