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cfr70 smoking bad

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I have an 2004 crf70 that I just can not get to stop smoking. So far I have had an engine shop replace the valve seals and clean up the valves, bored the jug 1mm oversize, installed a new piston and rings, and put back together with new gaskets and o-rings. This thing still smokes like before I started. When it was apart, we checked the head and jug for cracks and could not find anything. Twice when I pulled it apart, the cylinder walls and piston were covered with oil. I am stumped. Where is all the oil coming from???????? Is it possible there is a crack somewhere that is not visible?? I made sure to put the ring gaps spaced out and followed the manual for reassembly. Compression check right after the rebore was at 120 psi. I assume this will get better with some break in time. What else can I check???????????

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Did you put back all the right gaskets? If I remember correctly, there are some small o-ring type gaskets that could be easy to overlook. Also, you mentioned the valve work, but were the valve guides checked? If they're sloppy, it might make the seals wear out fast.

Did whoever sold you the piston give you the correct rings? How did the shop check for cracks? And what was the piston clearance when they were done? I know people who have had back experiences with machine shops.

Just throwing out some ideas. You've got a puzzle on your hands. Good luck.

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I used an aftermarket gasket kit from Moose. The 2 orings were not the exact shape as the originals so I bought those from Honda. The piston and rings came as a set, aftermarket though. Took the piston to the engine shop along with the jug; had good results with the shop before so dont think that is the problem. The same place did the head work. Is it possible that it will take alot of break in time to seat the new rings good? What is normal amount of smoking right after a rebore and ring job?

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My CRF50 didn't smoke at all when brand new. I don't know if there was some kind of run-in at the factory, though. I've rebuilt car engines and never had them smoke either when first fired up after the rebuild. Can't say this if this is typical or not, especially for motorcycles, but I wouldn't expect much.

How much time do you have on the engine after the rebuild?

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About 30 minutes of runtime. Starts easy and idles and sounds good but smokes like crazy.

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Double check to make sure the corrugated oil ring ends are not overlapped, allowing the oil to bypass and get up into the combustion chamber. Also check and see if the upper rings have a letter stamped near the end, the letter side is supposed to be facing UP. If those two rings are different colors, the flat black ring should be under the shiny ring.

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I ran the bike this weekend about 2 hours and the smoking is signicantly less. Im guessing that the rings needed more break in time. Will run more this coming weekend and hopefully everything will be back to normal.

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Double check to make sure the corrugated oil ring ends are not overlapped, allowing the oil to bypass and get up into the combustion chamber. Also check and see if the upper rings have a letter stamped near the end, the letter side is supposed to be facing UP. If those two rings are different colors, the flat black ring should be under the shiny ring.

Yup, what he said. Also possible the 'repair shop' has done a less than stellar job on assembly / boring.

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I ran the bike for about an hour a day for a couple weeks and the smoking gradually went away. It really ran well and started easy. I could also notice an increase in power from before the rebuild. Put it up for sale; first person in the door bought it. And now back to my 3 wheeler resto!!

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