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Discussion Starter #1
Gents

I bought this piece from someone who claimed they bought it with the intent of getting it running. They claimed they put a new piston and rings in it but can't get it to start because "it's back firing through the carb".

So I by it and start tearing it down. It does appear like the piston is new, and the cylinder is in very good shape. The piston size is stock, or 71.96mm. The cylinder is in very good shape as he said. The rings had 0.008" gap at the ends so they are very snug.

I put everything back together. I tried to start it and it did indeed backfire through the carb. Looking further I realized that the engine could be easily rotated by hand, virtually no compression. I am at a loss trying to understand why I have no compression. The rings and piston seem well matched to the clean bore. The crank seals should not have anything to do with compression??

I have not measured the compression, but you can turn the pulleys with almost no resistance. Also, when you put your finger over the plug hole there is NO compression resistance.

yet everything measured well??

Please help!

Thanks
 

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Updates. Crank seems to have no play. Both sides of engine do not appear to have any fuel or oil indicating a bad crank seal. The gaskets were all in good shape, the head did not appear to be warped.

I am stumped why the compression is so stinking low. The piston and rings were sized so well and the cylinder looked great?
 

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if the piston and rings are new looking as you say, then make sure it is installed correctly. should be a arrow or dot, something to show towards the exhaust.

did you install a new head gasket and top gasket, thats all i can think that would cause no compression besides cracks or warpage.

i have a repair manual i could email you if you like.
 

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No sorry, I did more work. I took the cylinder off and put it on a CMM. It's bad, oversize and majorly out of round. In fact, it took my machinist 0.015" till he got full cleanup!! I had to go 0.060 over to ensure it's cleaned up.

I guess my eyeballs are calibrated..

So cylinder is bored, with approx 0.0015" clearance. New wiseco piston, new gaskets.. hopefully she'll fire up!
 
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