lousy spark on an M

P and R Pete · Sep 11, 2013

Father-in-law"s M is stumbling, and his spark is a weak yellow. Couple of questions...

It has a ballast resistor. Any point to that on a 12-volt system? Though when we bypassed the resistor, there was no improvement.

It has new points/plugs/condenser. I tested the spark plug and coil-to-distributor wires for resistance. On my digital tester, the wires are showing no continuity at all, though with his analog tester, the needle goes halfway. Can that be right?

oldtanker · Sep 11, 2013

OK, on that system you are supposed to have copper core wires, not the newer carbon core. Check the coil to see if it's a 6 volt (requires ballast resistor) or 12 volt external resistor required or a true 12 volt coil. Make sure the gap is correct on both the plugs and points and that the distributer has a good ground. All electrical connections clean, tight and bright.

Rick

John T · Sep 11, 2013

If its a 12 volt system, it only needs a voltage dropping (12 to 6) ballast resistor if you use a 6 volt coil. If its a 12 volt coil and you still use a ballast THE SPARK WILL BE WEAK.

If its a 12 volt tractor with a 6 volt coil + ballast and you by pass the ballast, the coil will overheat and the points burn up quickly.

I have seen ballast resistors go bad and that can cause a weak or no spark.

A common cause of a weak spark is the points are burned or pitted or mis gapped or maybe the condensor is bad. I prefer wire core copper plug wires versus carbon core resistive supressor wires like cars use

John T

P and R Pete · Sep 11, 2013

Thanks for the input, gentlemen.

I don't know which type wires he has, I'll have to take a more critical look, but I do know they have a lot of resistance. He had a loose copper-core wire in his shed, and we put both meters on that, and there was no resistance at all.

But anyway, before I saw your posts, I went back up to his house and looked at the Champion D21s he put in, and the gaps were way too tight, so I re-gapped them to where they ought be. At the same time, I noticed his coil had the small wire from the positive coil terminal to the distributor, and believing it should be the negative side to the distributor, I swapped them around. After both fixes, it's running good. Funny thing is, I don't even know why it helped,

But I also need to see if it's a 12 volt coil, and whether it has an internal resistor or not, and get all this stuff right.

El Toro · Sep 11, 2013

The coil may be getting weak. Hal

John T · Sep 11, 2013

For the best spark the coil polarity needs to match the battery polarity. If its a POS ground the coils + wires to distributor but if its a NEG ground the coils - wires to distributor. A fresh set of properly gapped plugs can do wonders sometimes

John T

pete 23 · Sep 11, 2013

A weak yellow spark is kind of up to the interpretation of the guy checking it. If you check to a heavy painted or greasy surface it will not have a nice blue spark. Found that out the hard way when I was a kid on my own car. Was nothing wrong with the spark just the guy checking it. Me.

Also, your plug and coil wires will normally show open circuit if they are solid wire and have a poor connection between wire and the spark plug or dist end, but the high voltage will jump the gap and you may not know the difference when looking at the spark. If they show high resistance they are most likely resistor wires and it will effect the look of the spark. It will be a thinner spark so to speak.

lousy spark on an M

P and R Pete

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oldtanker

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John T

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P and R Pete

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El Toro

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John T

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pete 23

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