MF1655 (Snapper) Mystery Issue

[beetpickle]

Hi YT Forum,

My MF1655 (Onan B43M) starts well, runs for about 20mins then dies, backfires, and won't restart - yesterday it wouldn't even turn over. There was some white smoke when it did. Battery is fully charged, carb is clean, recently changed: fuel filter, fuel pump, condenser, coil, breakerpoints.

Any ideas?

Thanks,
Bill

 

[Janicholson]

Welcome to YT forums and ASAP parts. There is one thing I can think of that could be the issue. I assume that the engine is able to spin with the starter motor, (here we call that able to crank) if it actually runs we say is starts and runs.
A stuck valve can md that when it heats, the valve stem can warp and get stuck in the guide. a compression check will show near zero on one cylinder. Also hearing a change in the sound of cranking before starting is indicative of valve issues. Jim

 

[used red MN]

Hello bp, welcome to YT! My thinking is that we first need to know if it has spark. Have an extra spark plug widen the gap to 3/16” - 1/4,” plug it on one of the wires and lay or hold it against a metal part on the engine and crank the starter, any spark? A down and dirty compression test to check for zero compression is to hold your finger or thumb over the spark plug hole and crank it. If pulses of compression don’t push by your finger.. Houston we have a problem.

 

[kencombs]

Hi YT Forum,

My MF1655 (Onan B43M) starts well, runs for about 20mins then dies, backfires, and won't restart - yesterday it wouldn't even turn over. There was some white smoke when it did. Battery is fully charged, carb is clean, recently changed: fuel filter, fuel pump, condenser, coil, breakerpoints.

Any ideas?

Thanks,
Bill

If that red part means the starter wouldn't spin the engine over, it could be a loss of battery power to the ignition. The M series Onans are a simple battery, points and condenser engine. Weird series spark path but simple.

Just quitting with a backfire spells, condenser or coil to me. But not turning over may be an ignition switch, poor connection or bad ground.

I see you've changed the condenser. If you installed a new one, it is still suspect. Recently made condensers don't seem to be very reliable. Same is true of coils, but less so.

All that together, I would check the battery to coil wiring very carefully, with emphasis on the switch. edit: OR just connect a jumper from battery + to the coil and see if the problem gets fixed. If so, then do the detailed checks.