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Discussion Starter · #1 ·
Last summer I bought a replacement engine for my 68 CB350. The engine is from a 1971 but fit into the frame Ok. I finally got the engine installed and got it running last week. I put new plugs in it and I adjusted the valves and the timing. While adjusting the valves I found that the lock nut on the right exhaust adjuster screw was loose and the adjuster screw was out of alignment (the "T" was pointing the wrong way). I set everything correctly and the bike starts easily and runs pretty well...until the right spark plug fouls from being wet with oil.

It does not smoke, and the compression on the right cylinder is 170. The compression on the left cylinder is 160 so I suspect the compression on the right cylinder is higher due to carbon build-up.

My question is how do I systematically troubleshoot the problem; given that it has good compression and does not smoke?

Is it possible a ring is stuck since the engine was likely sitting for quite a while? The plug was already black when I bought the engine.

If its a stuck ring is there anything I can do to unstick it without removing the cylinders? Marvel Mystery Oil? Sea Foam?

Also, it if comes down to needing top end work, can I do it with the engine still in the frame? It was a pain lifting and maneuvering the engine at my age.

Thanks for any advice the forum can provide.
 

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An engine that has been sitting for an extended period and still has good compression and doesn't smoke likely doesn't have serious problems. I'd put in fresh oil and ride it a couple hundred miles, see if it clears up at least as long as it keeps running well. If the oil fouling persists I would repeat the compression test followed by a leakdown test. I would suspect a valve guide/seal before the rings.
 

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Are you sure it is oil on the plug? If it is getting oil on the plug it should be belching smoke. It could be black and wet from gas fouling.
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Discussion Starter · #4 ·
I let it warm up really well and it started smoking blue-white smoke from the right exhaust. Not a lot but definitely there. I'm going to pick up a leak-down tester tomorrow and see if I can nail down the problem. I've not used one before but my understanding is that I need to have the piston at TDC on the compression stroke so that both valves are closed. Then I should remove the inlet and exhaust valve caps and dipstick so I can listen for hissing noises to determine where the leak is. Does this sound correct? I've read that I should keep the pressure to around 30 psi to keep it from rotating the crank. Any other advice on this procedure?
 
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