Oil in the cylinder
 

I disassembled the cylinder of my 150cc GY6 last year to replace a number of leaking gaskets. The engine ran well but leaked too much. During teardown I noticed the piston was scorched on top so I replaced it and the rings. After a good cleaning everything was put back together with a new gasket set and it fired right up. However, after a few minutes running it started to smoke. No idea what color the smoke was as this was a few months ago.

So I just now got around to tearing it back down to find the cause. When I removed the cylinder head there was a small pool of oil above the piston and the gasket was wet with oil all the way around even though it wasn't leaking to the outside. Obviously the engine has been shifted around during removal so oil can travel up the timing chain cavity but I'm not sure how it would get into the cylinder.

I haven't pulled the cylinder off to remove the piston so the rings have not yet been checked. Rather I wanted to have an idea where to look before I went that far.

Has anyone seen this before? What should I look for as a possible cause?

Thanks!

Sounds like an air leak in the intake somewhere. Check all gaskets in the intake; manifold to engine, manifold to carb, carb to air box. Try some gasket sealer if gaskets show no damage.

I finally pulled the piston out to check the rings. The lower rings are not wider than the piston anymore. I'm guessing that is allowing oil to be pushed around the piston.

I ordered new rings and a new gasket set to put it back together. Hopefully that will take care of the issue.

When you put in a new piston and rings did you sand the glaze off the cylinder wall? The rings won't seat against the cylinder unless you do that. A new cylinder has micro grooves in it from the boring bar but a used cylinder doesn't. Had you kept running it the rings and cylinder would have seat eventually but it would have smoked for a while.

I did not sand the cylinder wall. Anything special I need to do in that process? Or other pointers for reassembly?

Thanks for the suggestion. I should be putting this back together this weekend.

Well, since you asked,

https://www.youtube.com/watch?v=f7JuMGsyDAg

I'm thinking a disk brake caliper cylinder hone might work for you. Have you priced a new cylinder?

Thanks everyone for the suggestions. I got the new rings in after lightly sanding the cylinder with some scotchbrite - I don't have a hone. The scooter is running and after the initial burning off of the lubricating oil it seems to be working well.

I'll give it a little more break-in time and then get a good ride in (once the rains passes). If the issue comes back I'll probably just replace the cylinder.

Thanks again!