CY hassle

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Guys and Girls!

Proud owner of MCD with 29cc CY engine and boy does it go.

Recently fried my piston ring :( and had to replace. Now once engine warmed up it increases and decreases rpm without any interference from me. Someone has suggested it is to do with 'shimming'. I'm fairly new to this great game - can anyone explain what I need to do (in simple terms) to rectify this?


sounds more like an Air leak to me. check your cylinder gasket. ? on the CY. is it the CY29RC motor with 2mm stroker crank and 35mm bore, or is it the CY made GP290 with the 28mm stock stroke crank and the 36mm bore?

the shimming suggested is typically for coversion motor going to 29cc and the 2mm stroke crank. cuz the crank is 2mm longer, the cylinder needs to be lifted with 2 cylinder gaskets to make clearance at TDC for the piston to complete a revolution without smacking the top of the cylinder.

if all you replaced was the ring, then i would be looking for a broken fuel line/return line, or an air leak at one of the gaskets.

start the motor, grab a can of WD-40. and with motor idleing, spray the cylinder gasket area, then the exhaust area, then the intake area with the WD-40. any change in rpms while your spraying the WD will indicate an air leak. once all your leaks are gone, then the idle should be stable and smooth.

Good Luck
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hi mate who did u purchase your 29cc from and sounds like an air leak or a hairline fracture in your cylinder and also check your isolator box isnt on upside down cause that causes the the same problem unconsistent rpm as i speak from experience
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