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I have a 2001 Foreman 450S. My oldest son just replaced the piston, rings, valves, valve guides and seals for me. Did not bore the cylinder as it looked ok and miked within specs. Smoking a lot again now. Checked compression today(3 times), showing 210 on compression check. Need some good grey haired advice!!
 

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I found that mine was still smoking after rebuild from residual oil that was in the exhaust and muffler
If smoked like a freight train before the rebuild so there was still lots of oil to burn off
You can ride it a bit and see if it clears up or rinse out exhaust with solvent
I rinsed my exhaust out 2-3 times and zero smoke after that
If its smoking more than before, you could potentially have an issue if something is still not right with the rebuild
Seems odd to replace piston if not going oversize unless it was damaged? Was it STD piston?
Was the bore at least honed to seat the new rings?
If its not residual oil in pipe, the smoking is from oil getting by the piston rings or valve stem/seals
your compression sounds spot on if you are disabling the automatic decompressor
Hope it just needs to burn off old oil-was it smoking pretty good before the rebuild?
 

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I found that mine was still smoking after rebuild from residual oil that was in the exhaust and muffler
If smoked like a freight train before the rebuild so there was still lots of oil to burn off
You can ride it a bit and see if it clears up or rinse out exhaust with solvent
I rinsed my exhaust out 2-3 times and zero smoke after that
If its smoking more than before, you could potentially have an issue if something is still not right with the rebuild
Seems odd to replace piston if not going oversize unless it was damaged? Was it STD piston?
Was the bore at least honed to seat the new rings?
If its not residual oil in pipe, the smoking is from oil getting by the piston rings or valve stem/seals
your compression sounds spot on if you are disabling the automatic decompressor
Hope it just needs to burn off old oil-was it smoking pretty good before the rebuild?
I am having to add oil so it is definitely using it. Spark plug is oil wet and black. We replaced the piston just because we had it torn down. The old piston was good. No cylinder wall scarring or scratches at all. It was smoking before the rebuild. I am thinking the head is warped or the valve seals have gotten brittle since it did get hot, not extremely hot. The fan did not operate properly so I put it back on a toggle switch. It runs great right now, just smokes a very lot. It has great compression.
I don't know if it has an automatic decompressor. When you use the pull rope instead of the electric start, it is plenty stout.
 

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I doubt that the seals are brittle, the new viton seals last a long time and are quite heat tolerant
I would double check piston skirt to cylinder clearances, you also might have ring ends lined up and getting blow by
or you could have pinched a lower oil ring on assembly, I have done it more than once
If you dont have a service manual you should come join us at atvhonda.com and download a free factory manual
MUCH better help over there and quicker answers-check it out
I only come over here once in a while to try and help out
 

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Did you hone the cylinder b4 installing piston & rings? Did you check the ring gap once installed? When the cylinder was checked, with micrometer, did he check it in several spots up and down the bore? Possible egg shaped cylinder bore. All or any of these could of caused the rings not to seat properly.
 

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Did you hone the cylinder b4 installing piston & rings? Did you check the ring gap once installed? When the cylinder was checked, with micrometer, did he check it in several spots up and down the bore? Possible egg shaped cylinder bore. All or any of these could of caused the rings not to seat properly.
Not honing the cylinder and/ or installing the middle piston ring upside down will cause excessive oil usage. The high compression reading that you are seeing is evidence of the middle ring “pumping” oil, which is exactly what it will do. A tear down is probably in order.
 
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