JR3 engine issue -

[1arunem]

Ok, I pulled the head.. Piston from the top looks fine. Pushrods fine, valves look ok, no obvious issues on visual inspection. (I have not removed them from head).

However, please look at the photos.. The combustion chamber has what appears to be a crack (possibly), between the valves..

Opinions please. Does this seem significant enough to cause my issue? I can't tell if it continues into the valve seat area. Thanks!

Not sure why they all posted sideways..

[meimk]

Maybe a silly question, but I was reminded while reading another thread: have you verified the carb throttle plate is opening? I've seen lots of Yamaha's that go dead when a throttle cable (usually the one from the pedal to the governor) breaks.

[1arunem]

It is opening, cable is good. Compression is down.

[Skoal82]

Is it a crack or just a casting mark?

[raydhd]

Originally Posted by 1arunem

Spun it over again and same results with gauge.. Poured some oil in the cylinder and gained about 25-30psi..

My understanding of compression tests IF the compression values increase with with the addition of oil that indicates piston rings as the oil acts to seal the cylinder to ring connection. IF it does not increase that indicates a valve or some other malady that the oil does not improve.

[1arunem]

Originally Posted by Skoal82

Is it a crack or just a casting mark?

Not sure but looks like a crack to me.

[Spicyunicorn]

Originally Posted by raydhd

My understanding of compression tests IF the compression values increase with with the addition of oil that indicates piston rings as the oil acts to seal the cylinder to ring connection. IF it does not increase that indicates a valve or some other malady that the oil does not improve.

You're correct I reread my post and said it backwards earlier in the thread

[1arunem]

Ok, after better inspection, found an issue with the head gasket. Mark on the head seems to be just a casting mark.

Replaced head gasket, torqued to 36ft lbs and checked compression with gauge. Had around 115psi.. Installed plug and carb, primed with a little fuel and spun engine over. Sounded down again. Checked with gauge and compression down to 25psi..

Any thoughts?

Pulled back apart but don't see anything obvious like issue with new gasket.

[1arunem]

Any thoughts? After surfacing head (piece of glass and sandpaper) and replacing with new headgasket, I had good compression. Even cranked up momentarily.

Then, went to spinning fast again. Checked compression and down to 25psi..

Any help/thoughts greatly appreciated. Thanks!

[cgtech]

Check for sticking valve's.