1939 Ford ignition caps

F

Fech

Member
Hello,
A "dumb" question. I am following the wire order as show in the back of the ignition caps, but apparently is not correct.
I found some diagrams but are some discrepancies (see attached).
Any advice is appreciated.
 

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The caps have the cylinder marked next to the spark plug wire socket (well, two cylinders marked, install for the side of the engine you are working on). The diagrams have the cylinders correct, right side is 1-4 and left side is 5-8.
I have never experienced a cap with the cylinder markings wrong. Please post a close up picture of the caps and we can compare to any we make have on hand.

Edit: staring carefully at the picture you posted of your distributor cap it appears to me everything is in the correct place. Reference I used to verify from Third Gen Auto site agrees with Van Pelt info, but is from opposite view.


What problem are you having with the engine running? It may not be a firing order problem.
 
Last edited:
Fech said:
Hello,
A "dumb" question. I am following the wire order as show in the back of the ignition caps, but apparently is not correct.
I found some diagrams but are some discrepancies (see attached).
Any advice is appreciated.
Click to expand...
510736


510737


510738
 
I connected as per caps numbers, but the engine is not running well, it has contra explosions.
 

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Hmm, that's a new one for me. My next guess was a 32-36 cap installed 90 degrees to correct (it can be done...), but you are picturing the correct caps.


As a quick diagnostics test can you leave one side off at a time and connect the other side then start the engine? It should run on 4 roughly but not pop back at idle. Whichever side is causing back firing needs more investigating.


Edit: a better idea if you have two people is to pull all 8 plugs and rest them on the engine where they will ground and connect all the plug wires. Hand brake on, transmission in neutral, turn ignition on, and have a second person turn the engine while you watch the spark plugs. They should spark in the proper firing order, 1-5-4-8-6-3-7-2. If that is correct then the problem is somewhere else.
 
Thank you for your quick reply!
I have basic mechanical knowledge, and
Is it possible to test the engine without the radiator? If so how long? I had to remove the radiator to remove the distributor
 
When the cap is on the left and fitted to the distributor the series of four numbers is read correctly, use these numbers. The second series of numbers will be up-side down. If the cap is fitted to the right side of the distributor the second series of numbers is now right side up, use these.
 
Fech said:
Is it possible to test the engine without the radiator? If so how long? I had to remove the radiator to remove the distributor
Click to expand...


Yes, you can run the engine for short periods of time, say 30 seconds maximum. You do want to keep these short and let the engine cool down between running, the cylinder walls get hot very quickly without coolant on the other side of them.


Do you have a 39 standard or truck? If yes, for space to remove the distributor it is easier to disconnect the battery (for safety when messing with generator wires),pull the fan off the generator, then pull the generator than it is to pull the radiator. Make sure to keep the generator lined up with the intake when tightening the big nut, if the generator is cocked when tightening it can break the ears off the front of the intake.


If you have a 39 Deluxe no need to pull anything else to remove the distributor.
 
A common failure point with the helmet distributors is the Spark travelling through the rotor to the central shaft. There are normally tell tale signs if this is happening.
 
Mart said:
A common failure point with the helmet distributors is the Spark travelling through the rotor to the central shaft. There are normally tell tale signs if this is happening.
Click to expand...
Have heard this can be a problem but have not experienced it. Mart, what are the tell tale signs?
 
Arcing through the rotor to the shaft typically leaves marks inside the rotor. If you look at the inside of the rotor you see grey marks. Also I sometimes see round marks on the brass where the carbon is when the arcing occurs. A simple solution is to wind electrical tape around the distributor shaft until the rotor barely slips on.
 
Ok. Thanks. What symptoms with the engine would I see if it's arching thru the rotor? No spark? Intermittent/erratic no spark? Only certain spark plugs not firing, but consistent not firing?
 
My experience with failing rotor arms and caps has been like this:

Car starts and drives great for a while.
Some time into the journey it starts to misfire at wider pedal angles. Drives ok at light pedal.
Gradually the threshold of pedal angle related misfire reduces until it just wont run at all.
Left long enough it will (sometimes) recover and I have been able to limp home in short steps after many cooling down periods.
The first time you feel a misfire at wider throttle turn around and head straight for home.

If you imagine the spark having to jump the plug gap, as long as that jump is the path of least resistance all is ok. The wider the pedal the more the compression pressure increases. As the pressure increases the energy required to jump the gap increases too. That is when the spark will try and find a weak spot in the system where the effort taken to go that way is lower that the effort taken to jump the plug gap.

Once that weaker path is found things will deteriorate quickly.

You can help matters by using low resistance plug wires (Solid core), non resistor plug caps and non resistor plugs. Plug gaps should also be conservative. (.025"). All these things help the situation by ensuring the spark has an easy time of jumping the plug gap.

If you have a really good rotor arm and cap you can use carbon core wires and maybe bump the plug gap to .027". Any steps in this direction will reveal any deficiency in the cap and rotor. I have used epoxy on caps and rotors (crab type) to help in known weak spots.

These problems are universal and independent of the distributor type. I have had problems with Helmet, Lucas and crab types.

All the above is from bitter personal experience and not just stuff I have read on the internet.

Mart
 
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