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Author Topic: Advice on rebuild  (Read 681 times)

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Offline bblass

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Advice on rebuild
« on: October 14, 2015, 10:07:48 AM »
Back in June I bought a non running dirtbike as a new project bike. I was able to get it running in pretty short order but always had an issue with it smoking and burning oil. This went on for a little while and since I never used it for anything other than teaching friends to ride and the smoke wasn't too terrible I let it be.

Eventually, I got a wild hair and decided I wanted her to be a reliable trail vehicle. After replacing the valves, valve springs, valve seals and piston rings she ran like a champ and purred like a kitten. I took her out on Sunday and beat the snot out of it with no issues, even taking it up on the highway for a few miles. When I got back home everything was running fine with the exception of needing to weld a new bracket for the headlight since the old one rattled off.

Sunday night I changed the oil and sure enough it started smoking again, much worse than it used to at that. The oil filter had some minor metal shavings in it but nothing too noteworthy. I checked the valves and they are all in spec save one intake valve being about .01MM loose, not enough to cause oil burning. Compression on the cold motor is about 135. Stock hot is supposed to be around 160 so I feel like that is pretty good. When it cranks from a cold start the smoke is almost immediate, not a slow roll like it dripping down from the top end.

Ran a leak down test last night and at 15PSI I have no leaks. Valves are all seated correctly and piston rings are doing their thing.

Here's the theory I'd like to run past you - Cylinder bore is slightly out of spec and fouled the new rings? That's why it ran fine after when it was hot from the ride sunday but once it cooled and contracted it smokes again.

That would explain good compression and the leak down test coming back as everything sealing correctly at TDC, right?

So would the wise course of action in this case be to have the bore sleeve checked and honed back to true then put in .25 to .5MM oversize rings in? If I go this route should I go ahead and replace/reseat the valves as well just to have the peace of mind of a completely new top end?

If anyone wants to come take a look in person they are more than welcome. I'm in homewood and happy to pay for diagnoses and opinions in beer. 

Offline bblass

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Re: Advice on rebuild
« Reply #1 on: October 16, 2015, 09:06:40 AM »
Figured it out last night! I measured the piston for my rings and not the bore :o... Rookie mistake but one that I hope I won't make again.

I put 73MM rings in a 73.25MM bore. Now to find a new set and get back on the road!

Offline Chitza

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Re: Advice on rebuild
« Reply #2 on: October 16, 2015, 09:29:59 AM »
Good catch! Hope you're on the road again soon!
Loud pipes make me hungry for Valium biscuits and scotch gravy. - kdtrull

Yeah....ham it up, crackers.   ;D -kdtrull
The politically correct term is "Saltine American". -KevinB