Trashed

Duffman

Member
Dec 29, 2018
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I'm pretty new to these 2 stroke motorized bicycles, but have developed a passion for them, but I've been plagued with some serious vacuum leaks using or should say reusing my not carburetor
My engines ran great, had 2 of them, one a straight 38 mm stroke engine, no mods, the other was a 40 mm, balanced crank pk80 type. I started experience a irregular idle then my engines wouldn't idle at all. I checked for leaks at the intake to cylinder and the carb to intake tube. I sprayed cab cleaner to detect the leak, then tried wrapping plumbers tape around the intake tube with limited success, the carb started leaking once again, checked the head gasket, jug base gasket, no leaks there. I also silicone sealed all around the carb to intake connection, but the leak prevailed. I decided that even though I had the leak sealed however briefly I still was suffering power loss. I even tried the racing carb, which seemed to have a better sealing capability, especially with the sleeve insert, but that carb leaked as well! I finally decided to pull the head and jug an discovered severe damage to the cylinder chrome coating of the cylinder. This damage was only above and below the intake and exhaust ports, slight scoring of the piston an no apparent damage to the rings. I will replace the cylinder and most likely the piston and rings as well, but Im very curious as to what caused the damage, I broke the motors in properly. Need some advice badly on how to proceed, thanks in advance!
 

FOG

Well-Known Member
Mar 3, 2019
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Usually the piston gets the damage first, not the cylinder, and especially not a plated cylinder, but the answer is still heat. When the piston gets too hot it expands to a size that's larger than the cylinder and wants to get stuck. I've seen dirtbike's seize so bad they locked up the back wheel.

Intake leaks can cause this by leaning out your fuel mixture. A light gray spark plug is an early warning sign of this. A hole melted in the top of the piston is an already too late sign! It happens.

One good way to check for leaks on a 2 stroke is to pressurize it. I usually use a rubber plug in the exhaust port and rig up a valve stem on the intake to shoot a small amount of air in. It should hold 5-10 lbs for 5 minutes or so. If it won't hold air at all it's still leaking. A spray bottle of soapy water will be your friend.

Good Luck and I hope that helps.
 

Duffman

Member
Dec 29, 2018
33
2
8
56
Thanks for your help. When I posted this I forgot to post my pics as well, which I will attempt to do so now. This damage is on the exhaust side of the cylinder as well and nowhere else in the cylinder.
1555801888445-1655390835.jpg
1555802198816959911841.jpg
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,621
432
83
Los Angeles, CA.
That cylinder is trash now, & you probably have all that flaked off chrome down inside your engine.

Clean out the engine case & install a new cylinder, piston & rings. (& be sure not to over torque the head back on).
 
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Duffman

Member
Dec 29, 2018
33
2
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56
Thanks guys, I'm definitely ready to replacing the cylinder, piston and rings. I want to flush the lower case and certainly would appreciate advice on how to do so.