Ignition Coil Questions - 1978 Fed Spec 40

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John Smith said:
I would start with swapping the coil and see what happens. Next would be either swap the igniter with a known good unit. Or pick up a GM HEI igniter and wire that up and see how it runs. I'm pretty sure you will find what is failing.
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Thanks John. I did order up a replacement coil, more on that below...
 
Coils...
I determined I had a 90919-02083 coil in the 40. I found what I thought was an OE replacement at a forklift supplier - it was not.

I can deal with that, but I thought I would check resistance of the primary and secondary windings. The secondary resistance is like 10x higher than that of the original coil! Resistance values of both orig and new are pictured below.

Is this to be expected for a new coil? Different winding count - I know this doesn't make sense to me either...
What does this do? Make a hotter spark? Work the igniter harder?
Should I try this new coil? Or is that asking for trouble?


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IMG_6839.JPG
 
I had a chance to exchange messages with @mattressking - I sure do appreciate his thoughts on my issues.

First, he noticed that I had de-smogged recently: and he asked what I had done to provide power to my fuel cut-off solenoid?
Answer - nothing. I had left it connected to the emissions computer. He said these can be affected by bad solder joints, and I would be better off finding a clean, switched 12v signal. Good idea!

Then, check fuel level in sight glass (is good), check vacuum, inspect plugs.

Next I brought up the header swap, and how @Pin_Head had mentioned a leak at the intake manifold can be no bueno, so we agreed that checking manifold for tightness, and carb mounting bolts would be a good idea. I like this idea after running her for 200 miles from 5k feet elevation to up over 10k, and then back down (cooling) and then back up again, and then shutting off for fuel. A few good heat cycles could have loosened things up.

So I think I will start with checking for tightness, checking plugs and then wiring the new fuel-cut power.

Then I might move on to checking all of my grounds at the coil and ignitor.

My '78 FSM has no ref to the electronic distributor, or how to troubleshoot the ignitor. If anyone has some references for diagnosing, I would be grateful.
 
Just to be clear, weak spark problems are the worst at full throttle, pedal to the metal, low RPM going up a hill in 4th gear conditions. This is the highest load on the engine. Vacuum leaks show up as a stumble when you transition from idling at a stop light to accelerate normally. This is because there is no fuel at the transition slot when you have a vacuum leak. With a vacuum leak, you need to increase the idle speed enough to pull fuel out of the main nozzle to get the engine to run at “idle”. Ignition problems are typically very reproducible. Intermittent fuel problems tend to be random.
 
Pin_Head said:
Just to be clear, weak spark problems are the worst at full throttle, pedal to the metal, low RPM going up a hill in 4th gear conditions. This is the highest load on the engine. Vacuum leaks show up as a stumble when you transition from idling at a stop light to accelerate normally. This is because there is no fuel at the transition slot when you have a vacuum leak. With a vacuum leak, you need to increase the idle speed enough to pull fuel out of the main nozzle to get the engine to run at “idle”. Ignition problems are typically very reproducible. Intermittent fuel problems tend to be random.
Click to expand...
Am I reading you correctly that you would be looking at fuel system then?

Any thoughts on the fuel cutoff solenoid?
 
Pin_Head said:
Just to be clear, weak spark problems are the worst at full throttle, pedal to the metal, low RPM going up a hill in 4th gear conditions. This is the highest load on the engine. Vacuum leaks show up as a stumble when you transition from idling at a stop light to accelerate normally. This is because there is no fuel at the transition slot when you have a vacuum leak. With a vacuum leak, you need to increase the idle speed enough to pull fuel out of the main nozzle to get the engine to run at “idle”. Ignition problems are typically very reproducible. Intermittent fuel problems tend to be random.
Click to expand...
Thinking about this some more, my issues seem to be most noticeable when I was running down the road and trying to just keep speed, so feathering the throttle.
Leaning on the throttle seemed to take away the issue.
 
Pin_Head said:
Chances are your problem is not a bad fuel filter.

The problem sounds intermittent and they can be hard to diagnose. Coils are very reliable, but they are also cheap, so you might try replacing it to see if the problem goes away. Igniter problems are more common than coil problems.

Weak spark ignition problems tend to get worse with increasing load on the engine because it takes a higher voltage to make a spark when there is more air compressed in the cylinder. Spark problems tend to be more regular or deterministic so it always misfires under the same operating conditions.

Fuel problems can be intermittent, like a piece of dirt floating around in the bowl that blocks the main jet, but this tends to be more stochastic or random.
Click to expand...
Agreed. I can think of maybe 3 or 4 times where replacing the fuel filter did not help. And then suddenly everything was fine!
I can't wait to start checking things out. I have some weekend stuff to knock out with the wife and kid first...
 
Pin_Head said:
The off idle stumble and hesitation on acceleration are classic signs of an intake manifold leak, which is a common problem on 2Fs. Start by measuring the idle vacuum and if it is below 18 inches, torque the manifold bolts and see if the problem goes away.

K, not sure if I know how to quote posts from other threads, but since I recently had a header installed, I thought I should check and make sure everything is snug and tight!

The carb mount closest to the master cylinder (outboard, rear) was not snug, so I was trying to tighten it up. It is a very short throw over by the master cylinder, so I got a set of stubby wrenches and tried to tighten. I never got there. The mounting stud either spun loose, or pulled out of the intake. So now I get to pull the carb and assess... :bang:
 
I spun the stud out and pulled the carb...

IMG_6883.JPG


I ran a die over the threads on the stud, but you can't run a tap down through the threads in the intake because the passage is closed! I ran a q-tip through w/ some cutting oil. Then some anti-seize on the stud and wondered if I could run the stud in and tighten? Or would it spin out?
I got lucky and got the stud in there nice and tight using the double-nut technique.

IMG_6889.JPG


By then the daylight was gone. Time to just put everything back on and mess with it another day.
 
Last edited:
Monday I fired the 40 up. She seemed to catch pretty quick!
I had drained the carb and run carb cleaner through everywhere I could.
I had just enough time to run down to the local Maverick and tank up w/ some ethanol free.
She took about 12 gallons - 3/4 of the tank. She ran well the entire time - no issues.
I thought I would see how the float level looked while running - plenty in the bowl!

IMG_6957.JPG
 
PabloCruise said:
Coils...
I determined I had a 90919-02083 coil in the 40. I found what I thought was an OE replacement at a forklift supplier - it was not.

I can deal with that, but I thought I would check resistance of the primary and secondary windings. The secondary resistance is like 10x higher than that of the original coil! Resistance values of both orig and new are pictured below.

Is this to be expected for a new coil? Different winding count - I know this doesn't make sense to me either...
What does this do? Make a hotter spark? Work the igniter harder?
Should I try this new coil? Or is that asking for trouble?


View attachment 3623142

View attachment 3623145

View attachment 3623146

View attachment 3623147
Click to expand...
Any thoughts here on my secondary resistance question?
I'm not willing to fry my ignitor by using the wrong coil.
I understand the principles of induction, but that is about it. I cannot come up with how this higher resistance for the new coil will affect spark.
If I can't figure this out I will simply return it.
Your thoughts welcome!
 
PabloCruise said:
Any thoughts here on my secondary resistance question?
I'm not willing to fry my ignitor by using the wrong coil.
I understand the principles of induction, but that is about it. I cannot come up with how this higher resistance for the new coil will affect spark.
If I can't figure this out I will simply return it.
Your thoughts welcome!
Click to expand...
The secondary resistance will not have a significant effect on the igniter. The primary resistance is much more important. If your check out coils, the secondary resistance varies enormously on different units, but the primary is always 1 or 2 ohms.
 
Pin_Head said:
The secondary resistance will not have a significant effect on the igniter. The primary resistance is much more important. If your check out coils, the secondary resistance varies enormously on different units, but the primary is always 1 or 2 ohms.
Click to expand...
Thanks Charlie!
I didn’t feel confident running these, so I made arrangements to return them.
I need to log some miles on the 40, but I suspect that the loose stud was not helping things.
 
I'd recommend reinstalling original coil, drive. Then carry the spare or buy 90919-02113 .

Then you'll actually be able to quantify failures a little more precisely on what you changed and when issues do/don't occur.
 
mattressking said:
I'd recommend reinstalling original coil, drive. Then carry the spare or buy 90919-02113 .

Then you'll actually be able to quantify failures a little more precisely on what you changed and when issues do/don't occur.
Click to expand...
Thanks Nate.

Do you know off the top of your head if this coil - 90919-02113 - would also work for a 3FE?
 
I don't want to poach the thread, but after reviewing the thread I have two "general" questions.
1. What is the igniter? On my '75 I have an external resistor on my coil and that's all. There are no other parts like the igniter mentioned in this thread.
2. Does the replacement coil 90919-02113 use an external resistor?

I am asking because I am preparing to do an engine swap and as I inherited the ignition system in this truck, I want to be sure it's ready for a proper 2F. ('78 2F on the way.)
 
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