DennisS

My Feedback: (40)

I have an older YS 120 that always ran like a top. It sat up for a while (lubricated) and now has alomost no compression. Thoughts, ideas appreciated.

ChuckW

Sounds like either a stuck valve or ring. I'd put my money on the stuck valve; it is common on engines that sit..

Pop the rocker cover off and see if one of them is stuck down. A little penetrating oil and pushing on them a few times will often free them up.

If that's not it, put a few drops of oil in the cylinder through the glow plug hole and turn it over a few times.

Gray Beard

My Feedback: (-1)

And you are never to use petroleum products in a YS, they play havoc with the diaphragms. At this point I would do what Chuck says then order up a couple of new diaphragms and gaskets for it. When something goes wrong with a YS and it starts running bad then the diaphragm is usually the problem.

ChuckW

I have one on the bench right now that I am rebuilding for someone. Several o-rings look like they've been eaten with some sort of chemical. Not sure what did it. some sort of cleaner maybe?

I recommended penetrating oil earlier for freeing up a stuck valve. A few drops of fuel might be a better option.

MikeL

My Feedback: (3)

Fuel is a much better option for a YS.  Like GB says, petroleum will eat the "soft" parts of a YS.  Chuck, it's possible that you're seeing the effects of using a petroleum-based after run oil.

ChuckW

Yeah, I'm not sure what got to it but something sure did. It wouldn't run worth a crap with all those leaks. It's all sealed up now, good as new and ready to give back to the owner.