jerryblewis

Member
Dec 3, 2005
3
0
Help!!

Bike: 1995 Honda XR250R

Problem: Smoking after top-end rebuild

History: Bike bought used in 2003. Bike overheated in mountains and began smoking. Had cylinder bored .5 mm and installed Wiseco forged piston; valve job and new valve seals and gaskets. Machine work was done by KTM shop (very reputable, does 90 % of machine work for other bike shops in town). Put about 50 miles on it one afternoon and bike begin to smoke again (looked like a two-stroke).

Tore it back down and found large carbon build up on piston. Checked the ring end gap, gap orientation, and oil ring ends (not overlapped). Then I took the jug, piston, and head back to the machine shop without disturbing anything and ask him to double check me. He found nothing wrong, so he re-honed the cylinder, and I reassembled with new rings and gaskets.

Well, you guessed it...it still smoked like a two-stroke. I talked to everyone I could think of and most pointed at some problem with the head. This week I pulled the head an took it to a different machine shop and had them die-test it for cracks, check for warpage, check the valve guides and install new valve seals (again). The only problem they found was the valve guide clearance was slightly greater than spec but they said it wasn't enough to cause smoking. One difference I should mention is that there was no carbon build-up on the piston this time. Yesterday I reassembled it and once more it started smoking.

I have checked and rechecked the valve-lash gap. I pulled the muffler to make sure the smoke wasn't just oil burning that was collected in the muffler. I drained the oil and refilled (Castrol 10-40) by measure (1.7 US qt.) to make sure it wasn't overfull. Perhaps one important factor I haven't mentioned is that it doesn't smoke when the motor is cold...it takes about 5 minutes of running before it starts to smoke. Also, the bike has about 50 miles on it since the second ring replacement. One thing I'm curious about is that the shop manual shows that the valve guides have O-rings on the bottom (they appear to reside in the valve port area above the combustion chamber). Could these be the problem?

Sorry for such a long description but was trying to anticipate any questions. I'm truly stumped! Would greatly appreciate "any" suggestions.

Jerry
 

nickyd

Member
Sep 22, 2004
873
0
did you change the valve stem seals?

also, you aren't running synthetic oil are you? that can wreak havoc when you are trying to get rings to seat after a rebuilt.
 

Tdel

Member
Mar 1, 2004
47
0
If I remember my 4 stroke inner working correctly there would only be a few ways oil could end up in the combustion chamber:
1 Rings Check
2 Valve seals Check
3 Head Gasket ???
4 Cyclender warpage at the head???
5 Cracked head Check
6 excessive oil pressure???
 

jerryblewis

Member
Dec 3, 2005
3
0
Thanks for the input guys. I'm running Castrol 10-40 (non-synthetic) and yes I did change the valve seals, twice. Tdel, you mention excessive oil pressure...do you know of anything that would cause excessive pressure on a XR 250 R?

Jerry
 

Tdel

Member
Mar 1, 2004
47
0
I am not sure how the valve train recieves its oil, but the return could be clogged, thus more oil entering than leaving. Did you replace the head and base gaskets upon each rebuild?
 

jerryblewis

Member
Dec 3, 2005
3
0
I used new gaskets for each rebuild. The machine shops installed the seals both times so I don't know if they did any deburring. The machine shop did a valve job but didn't install new valves.

Thanks,
Jerry
 

nickyd

Member
Sep 22, 2004
873
0
when you pulled the head - were the valves wet on the underside? i'm wondering if they didn't lap them correctly and/or you have oil getting past the seals. does the bike smell when its smoking like burning oil? thinking realistically - could the ring seal be bad? who did the top end? the machine shop? did they hone it correctly and then wash the hone residue off with SOAP and WATER (solvent won't do it) - if they left that residue behind, it would eat your rings FAST.
 

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