On an old Mercury 9.9 2-stroke, the model that twisting the grip both changes the throttle and shifts F-N-R, the motor idles fine and generally runs fantastic, except at may 10%-ish throttle, trolling speed. At that speed it will appear to be putting along just fine, then suddenly a big deep POOF sound, the motor jumps, a big cloud of smoke , the RPMs drop by about 1/2, then slowly builds the RPM back to where it was. This happens maybe once ever 30-60 seconds or so and it will do this over and over all day long. As best I can tell, I get the impression that somehow the gas in the crank case ignites momentarily. If you give it more than about 1/3 throttle it runs great, no issues at all. Has anyone seen this particular issue before and any suggestions what might be causing it? Could it be timing related?
