Thread: low compression 70cc

i'm working on a trx70 and it had around 140lbs of compression and i took the motor apart to see why it was so low and it had a ring gap of well over .20 inches so i bought some new rings and now its .18 inch on the ring gap and with a new head gasket i'm at 120lbs.......whats going on here....the head gasket seems thick

what does your service manual say the compression should be? i built a 88cc ran it a week and came up with 150psi.also broke in a china motor and came up with the same.that is what a good running ct should have.140 psi is damn near perfect!under 100psi i would think about a rebuild for the topend.P.C.

140 lbs of compresion is good, I'm guessing you have a stock cam in it. the larger the cam, you start droping stadic compresion. also you said you have .200 ring gap? I think you mean .020, .200 is over 3/16 of an inch. Since now you have droped to 120 lbs maybe the new head gasket is thicker.

yea .020...sorry.....i'll have to look at the book but i think it says it should be around 180psi -+10psi......and the head gasket is a little thicker

Originally Posted by linkandcuff

i'm working on a trx70 and it had around 140lbs of compression and i took the motor apart to see why it was so low and it had a ring gap of well over .20 inches so i bought some new rings and now its .18 inch on the ring gap and with a new head gasket i'm at 120lbs.......whats going on here....the head gasket seems thick

Some of you CT Drs. may correct me, but back in the day when I had an engine that exhibited low compression I did a *wet test* on cylinder or cylinders. That is squirt some oil in the cylinder and retest. If comp went up I generally had valve issues.

i'm going to hook up my air compressor to it and pump 50psi in to the engine and see where the air comes out

well after pumping some air into the cylinder on the compression stroke the exhaust valve is leaking pretty good.