I have a Chinese quad with the GY6 horizontal cylinder engine. It stopped running after several good years of use. I did a compression check which was around 50psi.....it had been smoking a little, so figure it's time for a top end rebuild. I ordered a full kit (head, piston, rings, gaskets, everything required to do the build). I pulled the motor, did the rebuild on my workbench, checked compression after the rebuild was complete and the engine was still sitting on my workbench with a result of 110-120 psi. I'm thinking all is well, so my son and I get the motor mounted back into the frame and all hooked up. Hit the start button and bam-a-lam!, she fired right up. While letting it idle, I proceeded to adjust the carb since it was idling a little high. I shut it off because I couldn't get my hand on the idle adjustment screw.

After a small adjustment to the idle screw, I went to start it back up. it would not start?? I checked fuel, checked spark and all was good. I checked compression......it was only 60psi?!?!?!?!?!

So my question is this? What else could cause this? I know I did the rebuild correctly (done plenty of them). Rings were aligned properly (ring gaps not lined up), timing correct, etc. I hate to have to buy another rebuild kit, but can't think of anything else it would be.

FYI, a full rebuild kit for the GY6 150cc is $39.95 + shipping.

Anyone have any ideas before I go for it again?

Thanks,
Roger

 

Do you have a leak in the head gasket? Did the rod bearing have any slop causing the top end to fail?

 

I really won't know until I pull it back out of the frame and take apart again. I blew some air into the cylinder port with my compressor and didn't hear the head gasket leaking I am thinking maybe a stuck ring. Thinking back, I think one of the rings when I installed on the piston head did not freely spin well. I stopped for tonight as I was quite P.O.'d. Figure I'll take it apart tomorrow after I'm calm again. I tried a little oil in the cylinder to test that theory, but didn't change anything. No unusual noises when it was running or when I tried starting again.

Probably something dead simple......figured I throw it out there to the masses for ideas before I go tearing it apart again.

Oh, forgot to mention, valve gaps were set to .004 on both.

 

.004 is a little tight on the valves. Take the air filter off and cover your carb with your hand.. its it sucking in pretty good? Is there any blow back?

 

Tore it back down today. Sure enough the top piston ring was wedged into the piston (stuck). I honed out the ring groove on the piston ever so slightly and put it all back together yet again...now sitting at 150 or so PSI.......man I tell ya, I can have this motor out the frame, tore down, back together, and back in the frame in less than an hour now.......Hoping this is the final time now.

As for the valve clearance gap of .004, that is what is suggested in GY6 manuals and many online resources. I thought it was a bit tight as well, so went to several sources to confirm. .004-.005 was the overwhelming recommendation.

Thanks for the suggestions folks...helped jog the old memory.

 

Glad you got it figured out.. if my top end goes, I plan on going big bore with big valve head.. I want more power lol