I have a Manco 90cc that tests out at 75psi, (seems low), my daughters 50cc almost compresses out at 100. I would think the 90 should have higher compression. Try new piston and rings? Help plz.

 

Put a few squirts of gear lube in the cylinder, turn it over a few times with the plug out, then check it. If it goes up, it's the rings. If not, probably a valve. May only need the valve clearance adjusted. Let us know what you find.

Denny

 

Went up a little over 5 psi, i took the cylinder off, chamber looks good, a lot of carbon on piston, I cleaned that off. Rings hard to find for these chinese atv's ?

 

It may have broke a ring. One of my Honda 125's did. Did you just take off the head or pull the jug, too? With the head off and cold, you should show a few thou clearance on the valves. They should be set with the piston on TDC on the compression stroke and the engine warm. Where to find rings? I don't know. Just found the supplier for Loncin parts the other day after a couple of weeks of serious searching. If it's a GY6 clone, parts are out there.

Denny