Yamaha 5HP 2-stroke low compression, bad idle

[nickpitcher]

My ~1990 Yamaha 5HP 2-stroke (model 5CS) has low compression at around 70-90psi. The service manual suggests it should be between 113-138psi. Additionally it does not idle well at all and stalls unless it's revved excessively. Not sure if that's related though.

I've cleaned out the carb and played around with the idle screw and air mixture screw for hours with little luck. So my next step was to check the compression. After discovering that it had pretty low compression I assumed that perhaps the head gasket had blown or there was an air leak around the head. However after removing all the covers around the cylinder I quickly discovered that it doesn't actually have a head, or a head gasket for that matter (that's a first for me). My next guess was that maybe the piston ring(s) had split, however I haven't had a chance to tear it down to find out.

Has anyone experienced this problem before? I'd basically just like to get it idling well.

Any help would be much appreciated. I'm thinking it could possibly also be caused by an air leak around the intake manifold?

Cheers,

Nick

[ausnoelm]

Lots of maybes here, is it still in pieces? a leak down test would have been nice, how did you clean the carb?

[nickpitcher]

It's back in one piece. I'd love to do a leak down test however I don't have the tools unfortunately. Cleaned the carb thoroughly with carb cleaner and compressed air.

[DennisG01]

I looked on SIM Yamaha's website - from '88 to '92 I didn't see mention of a 5HP?

I wouldn't worry about the compression numbers. It's 30 years old - it's going to have lower compression. Besides, are you POSITIVE that your compression tester is accurate?

Two cylinders? So one cylinder was 70 and the other was 90? The difference is more concerning than the actual numbers. But it may just be an "it is what it is" thing.

I'm not as experienced as others on here in terms of carbs, but it sounds like the idle circuit is the problem. Just using carb cleaner and air is NOT thoroughly cleaning the carb. Take it apart - needles, gaskets, etc. Take it all apart. Inspect EVERY little hole there is. Clean it better and replace any suspect gaskets and seals. You might consider an ultrasonic cleaner - I don't have one, but a lot of people seem to like them.

On the other hand, it might be worth an hour or two of labor to have a local pro look at it and get it running right. I'm not sure how much they run, but maybe source a new carb?