Posted by JDEM on September 10, 2019 at 13:01:58 from (47.33.231.52):
I am a little stumped with this machine I am working on. It is a 60s Harley Davidson. I have asked about this on a few Harley forums but gotten no useful information. Certain basics of battery ignition systems apply to tractors, cars, and motorcycles - so I figured I'd ask here for some thoughts.
Note - this is a two cylinder motorcycle with a 12 volt battery ignition. Has a distributor with no high-tension spark distribution. Just a single set of points but there are two high spots on the points cam instead of one. One bump for each cylinder. When this bike left the factory - it had one ignition coil with two spark-plug wire outlets (secondaries). Both cylinders get spark simultaneously. I guess this does not matter since one cylinder is not on the compression stroke at the same time as the other.
Here is my problem. The cylinder heads were modified to take two spark plugs each. So the engine now has two separate ignition coils and four spark plug wires.
I am trying to figure out what it the best way to wire the ignition coils . That if there IS a right way to do this. The coils have 4 ohm primaries and are marked 12 volts. Whoever installed this setup wired the two coil primaries in series which I find odd. Seems to me a 4 ohm, 12 volt coil needs full battery voltage to work well. With these two coils wired in series - the total primary resistance is 8 ohms and that must drop the battery voltage down a lot.
I have zero experience with a setup like this on a Harley. Generally speaking, 12 volt systems tend to work with 3-4 ohms of total primary resistance.
Note the bike did not run when I got it due to a melted piston. If I had gotten it and it started and ran fine - I'd leave well enough alone.
I am tempted to wire full battery power from the ignition switch directly to each coil. I have no idea how this effects the life of the points with the amp draw of two coils. I also do not know if I should be using a different ignition capacitor other then stock?
Kind of an interesting problem I guess. Cannot say I ever had to think about anything like this before. If this was a farm tractor with a 12 volt system and I ran 8 ohm resistance on the coil primary - I suspect it would have weak spark and be hard starting. So I am kind of thinking the same with this thing wired with two coils with the primaries in series.
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