Sticky valve.
I had a Honda CB1 400 4 cylinder bike that I was restoring. It had a 4 valve head. #3 cylinder had a sticky exhaust valve. First fire it up. It would run like crap. Then all of a sudden it would clear it's self out. I used a temp gun to check the exhaust headers. #3 was low, then come up to temp. When I rebuilt the motor. I find one exhaust valve with a minor bend in it. Eyeballing it. You couldn't see it. Put the valve in a set of V blocks with a dial indicator. It had less than .005" in run-out. Put that valve in another spot, and it wouldn't fit.
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