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Engine Ignition Wiring


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I have some general questions about dual ignition Mack gasoline engines and their operation.

We have a 1937 Model 75 pumper truck in our shop for a thorough restoration. The truck runs beautifully with a freshly rebuilt carburetor. Part of our job is to get a new set of wires on it.

My Mack experience is exclusively with diesel engines - so I am really at a loss here.

A) Do these engines always run on both ignitions simultaneously?

B) Does anyone know if an ignition wiring set is available for both the magneto and the distributor?

C) Is there any trick to threading the new spark plug wires through the original tubular wire guides? This looks like the toughest part of the whole project!

Thanks for your help.

Paul Van Scott

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The Twin ignition was meant as a back up in case of problems but like Albert said they run smoother with both firing and fuel economy is a tad better. If the old ignition cables are in the tubes try to use them to pull the new ones through, if not a "fish tape" and lube will get them through the tubes.

One important item , if the new ignition cables are Carbon Core you can easily damage them pulling them through the tubes


 “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’


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As the replys said the dual ignition was for reliability. The real trick is to get both systems timed exactly the same. Not too big of an issue if they are both distributors, but a magneto/distributor combination can be more problematic.

Because of the timing concerns the engines were generally started on "both" and run on "1" or "2" to prevent a mis-timed condition that can cause the piston to be burned. Assuming new plugs, points, etc. try running it on one or the other and see if it runs differently. If the idle speed increases on one or the other chances are the timing is slightly advanced on the fast runner.

I try to remember to run mine on alternating ignitions each time I start it.

Money, sex, and fire; everybody thinks everyone else is getting more than they are!

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