I did some tests, measurements and repairs to the instrument circuits on my 72 f250. Had cluster out and fiddled with wires on sensors.


Truck was running fine before this. Afterwards, it would start but ran extremely rough. Even in nuetral, the engine would barely rev. Smelled very rich, cough, sputter, ect. at WOT, rpms struggled to get to 2000-ish.
I checked wires, plugs. Results were inconsistent. I could pull of a given plug wire and it may or may not affect results. Plugs would spark if laying on fender. Verified firing order, it is correct. Cap and rotor are in good shape, look new.
Pulled all 8 plugs, 7 look good, one (#8) looks to have a mis (black but not oil fouled). Did a comp test on that hole and also #5, I get 100psi and it holds for a half hour. plug/wire combo #8 will make a spark.
Distributor was tight. I marked with punch, loosened bolt and tried rotating significantly in both directions. Not much change, barely runs.
Looked down carb while running, no fuel puddling. float seems OK.
Either my timing light is broken, or there is not enough energy to trigger it.


I decided to rebuild carb against better judgment, although it had a big flat spot. all seemed well inside. Afterwards it started once after bowl filled, and ran the same, like complete garbage. Now it won't start.
I measured all plug wires, they are in 8kohm range. Coil is an Accel, not sure how old. with KOEO I see 6V on + terminal, same as on starter solenoid. Given inline resistor this seems as expected. I believe this is the only wire needed to make the engine run.
The distributor has had points replaced with an electronic module. The vacuum advance appears to work as expect (I also tested this prior to this issue, and is the only thing I touched related to ignition).


I assume that this is issue is related to my work on the gauges, perhaps flexing the old wires or something. I am out of ideas, I hate to swap parts without a good reason. That said I would start with a new coil.



Edit: solution is in post #11 = PerTronix module was out of place and gap was to large.

 

If you have a points distributor, change the condenser.

Edit: I'm at work and was quickly trying to read your post and didn't initially see you mentioned having a spark module.

 

I have read and re-read your post. Clearly, you know what you are doing.
The smoking gun is the timing light that will not fire. You say you have an aftermarket coil of unknown history and a module. Something has failed to make the timing light not fire.
Compression is good, secondary wire resistance is as expected.
I would suggest these steps:
Buy or beg a timing light that you know works.
If no light, replace hot-rod coil with a stock unit and compatible module. Pertronix is the gold standard hereabouts.
Your problem smells like an ignition failure to me.

 

Well thank you for your kind words however after many hours of no success I'm not so sure!
Yes smells like un-burnt fuel... I can test my timing light (my riding mower is the only other thing I have with a plug wire) or borrow one easy enough. Thanks for the input.

 

You are fine, pal. Don't doubt yourself and don't go throwing parts at a problem where you don't trust your instruments.
Come on back in and talk to us. This old iron talks to us.

Semper Fi

 

I agree with ultraranger, I have been burned by this one time before and the engine would not rev at all but would run. Only reason I didnt swap this out when I was diagnosing my problem was the Oreilley's was out of condensers. I had never had a condenser go bad before this so I thought there was no way it could be that. Give it a go.

 

He said that he has an ignition module. No points, no condenser.

 

Make sure that the ground wire in the distributor is not broken or frayed. Also the + wire that feeds the coil, try by passing by going from the + battery to the coil.

 

The ignition module is a PerTronix.
The - coil voltage when key on is 1.1V
The + coil voltage when key on is 6.6V
The coil primary R = 1.5ohm (secondary 7k)
I=5.5/1.5 = 3.6 amps. I noticed the coil was warm when key was on during gauge testing. Power dissipated is 20 watts.
This is the 'closed' condition of the pertronix module which is just a switch triggered buy a magnet. It should be 'closed'unless it is firing the coil so these values are OK.

I found documents for what I think should be 1281 unit. says
2. Leaving the ignition “ON” with the engine “OFF” for anextended period could result in permanent damage to the Ignitor.
which is what I did while testing gauges, maybe 20 minutes or so.
The min voltage requirement is 8V

I wired pertronix red + wire directly to battery, same readings on the coil as expected.

The gap was way beyond the 0.030" spec. I pushed on the unit, it and the bar its connected to popped into place with 0.025" gap.

The carb pukes fuel now when cranking, which is why one shouldn't open a second can of worms. Still won't start. Next step to open carb back up, and test that pertronix switch will open.

 

Sounds to me like you did damage the igniter. I am assuming you are getting too weak of a spark for the engine to start and run correctly. Do you have a set of points and condenser there you can throw in it temporarily to test this theory?

 

Carb overflow issue was the float pivot clip wasn't clipped on the seat.


I tested the Pertronix module (I cheated and called Pertronix how to do it) and it works fine as I expected. Just connect pwr/gnd with test leads, wave magnet in front of it, and verify output (blk wire) switches. If observing - coil terminal, closed is 1v, open is 12V.


And?


It finally fired and runs. So the only thing I found wrong was the pertronix module gap, and sure enough that's what it was. I must have bumped it while inspecting the distributor. It had a gap more like 0.130" which resulted in random/erratic triggering.


While poking around under the hood I did find many, many other 'things' that need addressing, but that's life with an old truck.

 

Glad to hear this. And that it was cheap and easy.