Ignition question

[DWN]

My '27 Henderson Deluxe seems to have an ignition breakdown. When the engine is fully warmed, it seems to momentarily cut out under acceleration.

Last time I rode it, it was a warm (85f)evening, and after 5 miles of 25 mph riding, it was running poorly. I suspected fouled plugs as the exhaust is dark, and sure enough the plugs are black and sooty. It was running so bad I barely made it home.

I had Champion D-16 plugs in it, and have switched to D-18. I also have D-21 available.

Are my plugs too cold? Am I just running way too rich? Could the magneto be failing after it gets warm?

[PRG]

Howdy Rick,

Other items being ok - ignition wires, advance, mixture, etc.. - it could be a weak mag coil. Though not as obvious as, say /2 BMW's where the coil is bolted to the motor under a cover and heats quickly, the rise in heat as your engine and it warms may overtax a partly shorted/open windings in the coil.

Correspondingly, if it has weak magnets it will cutout under acceleration when cold as well. Every point of compression adds additional resistance to ground for the mag to push that charge though and peak cylinder pressure (max torque) occurs under acceleration making this situation worse. Your mag will either find an easier path to ground if it can - seen as sparks to various adjacent metal surfaces on Bosch/Lucas equiped machines - or in an effort to overcome this resistance/pressure, it will take longer to saturate and fire late and weakly resulting in partially burnt mixture and the false indication of a rich mixture.

If closing the gap on your plugs - I've had to reduce it as much as .006 to get an Indian 4 to run cleanly - eliminates the cutting out though the mixture still appears rich, then your magnets need recharging. Probably would be a good time for a coil rewind as well.

[T. Cotten]

I'll bet on vacuum leaks.

....Cotten