So my 75 F-100 w/416 has been running great since last year. New motor that had plenty of pep and pulled very good. Yesterday while hauling a load of wood she started missing, especially under load. Today, I started from scratch. Pulled all the plugs which looked very good, nice light ash color. Cleaned and regapped them. Replaced fuel filter (inline). Went for a ride and still missing, usually after a good warm up. I had a spare ignition module, it's a duraspark setup, replaced it and it seemed to run much better, but still not "quite right". I'm considering replacing the coil which looks like the original. The distributor is new as wires, etc. On the coil bracket is, what looks like, a condensor. Anyone know what part this is? a resistor? I'd like to replace it as well but I want to make sure they are matched up. Any help appreciated. Thanks!

 

It is. It is used for radio noise suppression.

 

possibly a cracked distributor cap? have you checked your timing?

 

Yes, played with timing a little bit, keeping between 12-15 degrees. Cap was new last year. It's just odd that it had been running so nice and suddenly started missing. It would idle nice and you could give it gas at idle with no problem. Under acceleration, and shifting through the gears it would stumble. It's much better since I swapped out the module but still not "snapping" like it used too.

 

If you hook a vacuum gauge to it what does it show @ idle ? does the needle fluctuate or hold steady ?

 

redroad is onto something, make sure the vacuum is night and steady, otherwise it might be a bad valve, or maybe a vacuum leak at the intake manifold-to-head interface.

If you pull the coil wire from the distributor, hold it an inch away from the engine block and crank the engine over, does it have a nice big spark?

 

Just checked vacuum at intake manifold. Had 18# @ idle. Needle was very steady with just a hint of movement. Idle was very smooth.

 

Still at it......new plugs, new coil, new ignition module. Starts and idles perfect. Will run smooth if you go through the gears easy. When I accelerate hard, then it misses and bucks a little bit. Same if under load, hauling load of wood, etc. Vacuum 18" at idle. When I rebuilt the carb last year I used the standard kit with power valve. I checked another kit I have and see the valve is a 4.5. I usually don't run the truck hard but I did "crack it open" just prior to all this happening, and it seemed to happen overnight. Could I have blown the power valve or have the wrong size vavle installed? It's a 4100 and has run flawless for the past year. I'm starting to run out of ideas, any help appreciated. Terry

 

Fuel delivery under load sounds like something perfectly reasonable to suspect .. There are dual rated power valves also .. I forget if you said it but have you adjusted the vacuum advance if it's adjustable ? ..

 

well................that's a good question. The distributor was a remanufactured unit that I sent out to have set up for my application, timing, cam, weight, etc. I believe it to be the stock distributor found with the duraspark set-up in 1975. Is it adjustable ??? I really don't know.

 

It should be .. You are describing something that did not exist previously and only has appeared recently, is that correct?

 

Yes, this problem seemed to happen overnight. I had used the truck to haul some items. I "opened it up" on the way home (unloaded) and parked it. It sat for a couple days and when I started it back up it started running as stated. Like I mentioned, it ran super prior to this problem.

 

Inspect the carb carefully to make sure non of the plastic parts have deformed or melted due to heat .. Make sure the choke is operating as it should to the fully open position .. What carb are you running ?

 

I'm running a autolite 4100/1.12. It was originally from a early 390 galaxie 4spd car. I figured it was a good fit for my set-up. I put a kit in it last summer. One concern I have is reading that the power valve number should be 1/2 of the vacuum reading at idle. I think the PV currently is around 4.5 and I have 18" vacuum @ idle. As stated, it ran very smooth up until a couple days ago. Thanks for taking the time to help me out! Terry

 

Also, I don't run the choke. This truck is only driven in summer, I have disconnected the choke mechanism. Other than a brief stall on initial start up it warms up fast and idles nice.