New Top End - Low Compression

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08-27-2018, 12:47 PM

Drosner

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New Top End - Low Compression

Have a 30 year old 250cc Polaris Trail Boss 2 cycle and had the cylinder bored and a new Wiseco piston and rings added. First test I couldn't get higher than 75 PSI on a brand new compression tester. So I assumed that the rings moved off their pins when I installed the cylinder and took everything apart, double checked the clearances, ring gap, etc. Installed brand new gaskets and re-torqued to spec and getting 75PSI and sometimes if I pull hard almost to 100PSI. I'm holding full throttle when I am testing.

Would the crank bearings or seals impact the compression? I've got the exhaust and air box off - but would assume that helps improve compression? I've also got the nipple where the old goes into the engine open if that could impact anything...looking for a plug for that...

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08-27-2018, 05:26 PM

LT80

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Does it run?
If so, run it a while to seat the rings. Comp will prolly come up.

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12-02-2018, 09:01 PM

Drosner

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Since I’ve posted I’ve got it to run and it’s been running great since. Took a few carb cleanings as I had some junk in my gas tank that was clogging it.

I havent tried a new compression test since all is well. Strange that I was getting less than 100 psi and now it runs really well.

Thanks for the advice though!

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