compression minimum?

[Checkride]

On aircraft we remove the spark plug; squirt Marvel Mistry Oil in the cylinder (about 4oz); install the plug and turn over the engine by hand; when you feel the compression stroke coming up turn over real slowly; you may need to do this 10+ times; let it sit and soak while you drink a cup of coffee then repeat the process. Works wonders on stuck rings.

[Checkride]

Also, The Service Manual says at sea level Standard 178; Minimum 142 & Maximum 199.

[scottyb]

Originally Posted by Checkride

Also, The Service Manual says at sea level Standard 178; Minimum 142 & Maximum 199.

So those are the Yamaha service limits for this motor, then? Thank you Checkride

[Checkride]

Another thought of the evening, a little carbon buildup on a valve or valve seat will cause loss of compression. Can't always blame it on the rings. However, the marvel mystery oil trick will clean up carbon, too.

[Flintrock]

Sounds like the poor 'ol girl been rode hard and put away wet......alot. I would guess the bottom end is gettin ready to let go. Time to tear it down and find out.

[sho305]

Its not that hard to pop the head and check/lap the valves. If oil didn't help then it might not be rings might be valves....far as low compression. Still could be a cam timing or wear issue though. One thing you can try is take the carb hose off and fix the throttle open (ignition off of course) and if you can take the muffler loose from the head. Then turn the engine by hand to the compression stroke, turn it and listen to valve leakage. For a lawn engine with comp release on the cam you have to loosen the intake lash so it stops working, but I think most yamaha retard the timing you have to find that out for whatever engine it is. But if you hear a lot of leakage out the port as you turn it back/forth on comp stroke then the valve is not sealing. Leaking head gasket could also be cause check for that first.