250-500cc Low compression on '93 360

[brian bush]

I just want to bounce this off you guys in case I am missing something here. My '93 360 has low (about 115psi) compression. Hard to start - only starts with a wide open throttle! Kicks over way to easy for a 360, much easier than my newer 2002 360, more like a 125 or 200. So I took off the cylinder to inspect last weekend. Here is what I found. The cylinder looks beautiful! Still has some cross hatch in there on the cast iron liner. The piston looks nearly new. I expected a lot of wear, but the wear on the piston is minimal. It looks great! The end gap on the rings measured .012" and .013"at the top and bottom of the cylinder, so I think that is all good. I am wondering where I am losing compression?

The head gasket seemed to be sealing very well and the base gasket also seemed to be sealing very well. There was no evidence of any leaks on either. All of the head and cylinder bolts seemed to be properly torqued down.

The reeds are not sealing well, and I will replace those. That surely is contributing to the hard starting but I can't see the reeds causing low compression. Also, I don't believe it is burning any transmission oil. The spark plug looks perfect - light brown and not oily, so I don't think the wet side crank seal is the culprit. Can't detect any evidence of an air leak on the dry side crank seal either. It ran pretty nice once I got it started. I am kinda stumped. Any thoughts on what I might be dealing with?

 

[rancher1]

I am surprised it has a cast iron liner.

 

[ohmygewd]

Best way to really diagnose is to seal up the exhaust and intake and perform a leakdown test compress air and a gauge. Comp tester can be deceiving if the things aren't screwed tight (use some teflon tape). Also, hold it wide open and kick it 3-4 four times in a row and the gauge should give you the highest reading.

The thing shouldn't have a cast iron liner maybe confused with the nikasel liner?

 

[bulto]

sounds like you had stuck ring. Throw some new ones in after you clean everything up.

 

[justintendo]

im wondering as well, you must kick wot until the gauge quits climbing.

i have a sleeved 360 cylinder as well on my shelf.

 

[brian bush]

OK, I hadn't been to where this bike is stored for the winter but I think I have figured this one out now. Even though the rings are well within spec, I believe the piston is undersize and that I will need a b or c sized piston to compensate. Even though the piston that's in there is in really good shape, I swore I could hear a little piston slap. Now it is all starting to make sense to me!

 

[2premo]

OK, I hadn't been to where this bike is stored for the winter but I think I have figured this one out now. Even though the rings are well within spec, I believe the piston is undersize and that I will need a b or c sized piston to compensate. Even though the piston that's in there is in really good shape, I swore I could hear a little piston slap. Now it is all starting to make sense to me!

the one odd sound mine made sounded like a piston rattle, pretty sure it was the power valve loose in its bore
talked to another person about it he sleeved his holes due to wear, this was also a 93
not positive thats what you're hearing or experiencing

 

[PawPaw]

Have you checked the exhaust pipe? If stored as stated it could have a bunch of crap in it or maybe it has come from together. At any point, if the exhaust is not free and 100% open it will effect the compression. You can check this by putting the engine back together and then do the compression testing both with and without the pipe on it. If the numbers are higher with out the pipe, then you have found your issue.

Paw Paw

 

[rancher1]

The B or C piston will make very little difference.

 

[brian bush]

I appreciate the input guys. I will check the compression with and without the expansion chamber PawPaw. I could also put new rings in it in case it is a stuck ring as bulto mentioned.. A couple of updates. 1) it is actually a standard nikisil bore. I was mistaken about the cast liner from low light conditions and an untrained eye. It is literally in perfect condition though, with nice cross hatch and the ring end gap was well within spec. 2) Last time I was up there, I put everything together, with a new base gasket and head gasket o-rings. I didn't notice any restrictions in the pipe but it is something I should double check. Still feels like it has very low compression although I didn't take the time to put a gauge on it when I was up there.. Perhaps a leak down test is in order as ohmygewd mentioned. Still scratching my head though as my understanding is that most of the compression in a 2 stroke takes place above the exhaust port and everything seemed to be ship shape.
I guess I could also do a squish test with solder to see if something is way out of wack there. I suppose someone could have modified the cylinder head/combustion chamber in some way that is affecting things?

 

[brian bush]

Do you have any thoughts rancher1?

 

[rancher1]

How thick was the base gasket, there were a lot of combos in the 1990s.

 

[brian bush]

It is a 5mm base gasket. Just finally able to check..