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!
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!