New Guy Here - SPX Gangg (w/ low compression)

[natek]

Hey all. New guy here. I just picked up a couple SPX (94 & 95) and I've been learning a lot. I'm a first time owner but not first time rider. Very much there is a difference.

My 95 is good. 140ish psi on both cylinders.

My 94 is not good. 135ish on front, like f'n 30 on rear.

But the piston doesn't look destroyed. The ring doesn't look super warped. My question is : is there ANYTHING that could cause a low compression in only one cylinder that I am unaware of? I firmly believe in devoting oneself 110% to learning about new projects, and have absorbed more jetski knowledge in the last two weeks than I've ever known.

Photos attached for intro / diag.

 

[burtshaver2021]

I’ve seen low compression in one cylinder, once I took the cylinder off the rectangular ring was stuck in the ring groove, once I got the ring out I measured the ring end gap and it was out of spec but the cylinder was all within spec for wear, taper and out of round. Not saying that’s the case here but it is a possibility

 

[burtshaver2021]

Post a picture of the cylinder dome for the cylinder with low compression

 

[mikidymac]

The dome won’t tell much. We need to see the sides of the piston.
But that piston crown looks lean. Usually the cause is carbs or fuel system and result is seizures and low compression.

 

[natek]

I'll try to post up both, every time I've seen low compression anywhere on the forum it's always chunks missing out of the piston head. I'll try to pull the cylinders today or tomorrow

 

[natek]

Post a picture of the cylinder dome for the cylinder with low compression

I don't believe I am getting spark in the good cylinder so some fuel mix seems to be there but that's about the only difference I notice. Going to pull the cylinder off today to get a look at the side of the piston itself

 

[burtshaver2021]

I don't believe I am getting spark in the good cylinder so some fuel mix seems to be there but that's about the only difference I notice. Going to pull the cylinder off today to get a look at the side of the piston itself

View attachment 65751View attachment 65752

I don’t see any issue with the dome, but I’m not very good at reading pistons, and such, relatively new to all this. Like you said, take the cylinder off, look for any scoring or scratches that can be caught with your finger nail, if everything looks good, measure the bores to see if they are within spec and go from there.

 

[natek]

The dome won’t tell much. We need to see the sides of the piston.
But that piston crown looks lean. Usually the cause is carbs or fuel system and result is seizures and low compression.

How can the carbs cause a low compression situation? I'm new to both two-stroke and to carbs at that

 

[burtshaver2021]

How can the carbs cause a low compression situation? I'm new to both two-stroke and to carbs at that

If the engine is getting to much air instead of fuel then it will burn much hotter, burning off the lubrication. Once you get it figured out, you should go over a few things to find the underlying issue that caused the problem, carb settings, leaking carb gasket, leaking crank seal ( do a leak down test ) or I’ve heard of people spraying carb cleaner or starting fluid by the crank seal to see if rpms go up, but myself I’ve always just done a leak down test. Fuel system sucking air ( again pressure test fuel system )

 

[natek]

If the engine is getting to much air instead of fuel then it will burn much hotter, burning off the lubrication. Once you get it figured out, you should go over a few things to find the underlying issue that caused the problem, carb settings, leaking carb gasket, leaking crank seal ( do a leak down test ) or I’ve heard of people spraying carb cleaner or starting fluid by the crank seal to see if rpms go up, but myself I’ve always just done a leak down test. Fuel system sucking air ( again pressure test fuel system )

I know for a fact the crank seal is bad. Oil all in the flywheel cover and in the bottom channel of the ski hull. I already figured this one was going to be a rebuild anyway for that, But now I want to know if the compression is related

 

[natek]

I think I may have found the issue as a faulty ring or piston out of round. At top dead center there is no side to side play in the good piston/cylinder.

At TDC on the bad cylinder there is about 1/32" approximate side to side play.

 

[burtshaver2021]

I think I may have found the issue as a faulty ring or piston out of round. At top dead center there is no side to side play in the good piston/cylinder.

At TDC on the bad cylinder there is about 1/32" approximate side to side play.

Yes a bad crank seal would be related to low compression. If you have oil leaking out a cranshaft end seal then I would say your inner crank seals may be bad as well, there shouldn’t be any standing oil in the crankcase, only in the rotary cavity. To know if your inner crankseal is leaking as well, you would need to pressure test the rotary valve cavity. No way to change the inner seals without having the crank rebuilt, or you could just instal a shut off valve on the rotary oil feed line. Yes, that side to side movement could be just the rings or could be a worn cylinder, only way to know is to measure