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Biker
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stalls out when warm

Since the carbs have been all cleaned out w/ new jets and nice clean air filters, + the fact she goes fast enough to shift her into 4 gear. . .

It CAN be a bad coil. Having worked on the larger size gas engines for roto tillers and generators , I have found that a magneto coil can go bad- sort of. At an idle and about halfway up, the engine ran fine. The carb was brand new so I put in a new magneto coil in and the engine worked as it should.

Sometimes with a weak coil, the insulation is not what it should be and voltage jumps around. A hi voltage magneto coil is not much different than a battery ignition coil, both have a primary and secondary winding. The magneto coil is mounted or wound on a laminated iron core that allows a flywheel magnet to pass over or under it to make the current for the primary which gets opened an closed through points with a condenser across the points to absorb the back voltage when the points open.

ignition coils are very similar to a magneto coil, having a laminated core or a core of heavy iron wires and has both ends of the low voltage or
primary winding available. Bat+ goes into one side of the coil, other side of coil goes to the 'points an condenser' and of course bat- is grounded. Sometimes if both sides of a magneto coil's primary is available, I have wired the magneto coil up like an ignition coil because flywheel magnets were weak and the engine ran great. SO yes it can be a bad ignition coil at higher engine speeds. If one does not know a coil can go bad, ( because the engine does run ) the engine when run at higher speeds will start to **** out as if from lack of fuel, when in fact it is starving for a spark.
 

Biker
Joined
1,756 Posts
I just replaced the coils to be amazed at the lack (none) of spurting and sputtering and it had lots before. I vote check coils too and wires and caps. Maybe try trimming the wire to caps also?
So true Aphrodite, ignition magneto's, step up transformers that sometimes work with low voltage mags and ignition coils, anything to make a spark.


Sometimes the insulation goes bad and voltage arcs inside the Hi voltage coil, might run good at lower speeds but higher speeds, and higher voltages, these coils just won't work well, most of the time leading a person to think it is a 'fuel' issue not knowing coil will **** out at higher engine speeds. Sometimes putting a coil in a low temp oven for a few hours WILL DRY it OUT. in a high voltage or ignition magneto, either the hi-voltage secondary goes bad, dampness is one reason, or maybe magnets are weak. a bad condenser will cause a mag to either have no spark or a weak spark or a good spark at lower engine speeds. I know this through experience.

Also there are companies that can re-strengthen magnets for magneto's.
 

Biker
Joined
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So I had some time to test out the coils. Both coils read at 1ohm on primary and 0 (nothing) on secondary. Also checked for spark and get very good spark from both cylinders.

so wondering if she is possibly catching air at hiring speeds, possibly leaning out the mixture.
.

Did you check for air leak around intake manifolds, spray carb cleaner where manifold bolts to engine an carb/s if idle goes up you have a leak.
 
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