It's a late 1970's Honda BF75, 4 stroke. Serial BF751007536. Runs rough at idle, stalls with any throttle.

Pulled carb apart, cleaned thoroughly, made sure jet was free of debris. Reassembled. Still rough at idle, stall with throttle.

Removed the spark plug wire from the lower cylinder and there is no difference in roughness - lower cylinder isn't firing for some reason, but upper is. (Removing the spark plug wire from the upper cylinder immediately kills the engine.)

Swapped spark plug wires. Same result. Swapped plugs. Same result. Tested for spark at both cylinders, appears to be just fine.

Figured it was a compression issue or stuck valve. Pulled off head, pulled out valves, cleaned all thoroughly. Replaced valves. Timing and movement of the valves seem to be correct. Checked point with a voltmeter, seemed to be making good contact at both positions as it should. Replaced head gasket and manifold gaskets, reassembled motor.

Same result, lower cylinder not firing.

Tested compression levels, both cylinders check out fine.

What am I missing here?

I know the spark is shared between the two cylinders, so could it be that they are both sparking for the first cylinder's firing, but not for the second cylinders firing? Like I said, I inspected and tested the point and it seemed fine, showing good continuity for both timing points. But...is there a chance that there is no voltage available to the coil at the time of the second cylinders firing? Is there a possibility of a weak magnet in the flywheel, or the primary coil is bad?

Or point me toward something else to look for. Thanks!

So... an update. I disconnected the exciter coil and wired the charge coil in it's place. Now Cylinder 2 is firing, cylinder 1 is not. This coil is on the opposite side of the flywheel, so timing wise that may make sense. I pulled the flywheel off, all the magnets seem to be strong. Visually the coils look fine. Put it back together. Wired it back the way it should be.

Pulled the cam shaft gear off, inspected the centrifugal timing adjuster. Visually it looked OK and seemed to operate fine. Put it back together. Widened the breaker point gap. Engine wouldn't start. Tightened the breaker point gap, and the engine came to life. Both pistons are firing now. But.... there are quite a few partial backfires so timing seems to be off some still.

So... Any thoughts out there? Is there a way to measure the centrifugal timing advance unit, or should it just be replaced? What is the gap that should exist at the breaker point? (I don't have a manual, anyone have that measurement handy?) Is this possibly a sign of a bad condenser?

Thanks for the help!

could be worn points or bad coil or bad flywheel