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1979 CB750 Problems with Cylinder #3

If you have no experience with engine internals then I would suggest finding a used motor and swap them out.
 
Heh,

Check the valve clearance.


Ironic, because I just had a low compression #3 but was because the intake valves were out of spec.
 
If you dont mind doing the work, I'd pull the engine and remove the head. Take the head to a reputable machine shop and have the oil seals replaced and valves gone through and setup. While the head's off, get a new set of rings and inspect the pistons for cracks. You can also take the jugs to a machine shop and have the cylinders honed and checked for cracks, etc. When you reassemble it, it's easier to check and adjust the valve clearances while the engine is still out of the bike. This process will most assuredly get the compression back to where it needs to be.
 
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