After a complete rebuild on my 250 trailblazer I'm only getting 90 psi. The cylinder has been bored .040 over and the head shaved .020. I've pulled the jug to make sure I didn't damage a ring and all looks fine; no scuffs, scratches, or other unusual marks. I have rechecked the machinist work and there is .003 sidewall clearance between the piston and cylinder. The ring end gap is good at .008. I'm at a loss as to what the problem could be. I thought that the head could be cracked and swapped it, still no luck! Anybody been down this road before? What am I missing? Maybe it's time to get drunk instead.

Thanks!

 

You didn't get a flat top piston by mistake did you? Did you check compression with the same gauge before the rebuild, what was the reading then? What type of gauge is it? The schrader valve needs to be in the fitting that screws into the spark plug hole so your not adding cyl volume with the gauge hose, this would cause a lower than normal reading.

 

No, It's dome top piston (Wiseco to be exact). And the schrader is at the end entering the head. The gauge shows 130 on my other 250 Polaris so I'm pretty sure it's close. I've never had a rebuild come out like this and it's frustrating to say the least!

Thanks again, any other suggestions?

 

You did have the pipe on it when you tested right?

 

I've tried it on and off. The engine will fire and try to run, but as soon as the starter is released it's over.

Thanks

 

You need to have the pipe on, has the cyl been ported at all?

 

No prting has been done. Just bored oversize. The pipe should have little to no affect on a compression test, right?

Thanks

 

Add oil to the spark plug hole, a teaspoon or so, and try the compreeion check again, if it rises then you have some kind of ring issue. Was the cylinder cross hatched properly?? You may have accidentally installed the rings upside down or accidentally broken one during installation, it is easy to do.

 

The crosshatch pattern looks good and the correct finishing brush was used (per Wiseco instrctions) According to the sheet that comes with the piston the rings do not have a top or bottoms side (ive tried them both ways anyhow). A wet and dry test reveal clos to the same reading (slight difference for the space that the oil had displaced). I am beginning to wonder about the crankcase sealing. It's the only thing I haven't torn down again for inspection.

Thanks for all the contributions!

 

Yep, I think that would be the next step. The base gaskets are known to mess up and cause problems if it is not a Polaris gasket. The aftermarkets are known to not be as good as the Polaris ones. I would check the base gasket really well and look for any leaks in the crankcase. Did you replace your oil seals in the crankcase? Could cause it also. Good luck.