The truck is the 77 in my sig. A while back I noticed this condition and, at the same time, a fuel leak from the rubber hose between the fuel pump and steel line from the fuel tank. The hose was cracked and leaking fuel. I figured the skipping was from the fuel pump sucking air through the cracks in the hose. Replaced the hose and the problem remains (just not leaking fuel now). Fuel pump is good. Fuel filter recently replaced. New plugs, pcv valve and breather filter recently.

Symptoms - skipping/bucking only happens during acceleration. At cruise it does not happen or is not noticeable.

I do not think it is ignition related as it is not a constant, unless the vacuum and/or centrifugal advance in the distributor is not working correctly.

My thoughts as of this time:

Accelerator Pump Diaphragm Bad (haven't checked yet but will this afternoon)

Rubber Hose From Fuel Tank to Steel Line (was under truck and hose seemed spongy - possible collapse when more fuel is needed?)

There are no fuel leaks under the truck. Outside of the above thoughts, I don't have any other ideas besides attacking the ignition and it doesn't seem like ignition.

Any additional ideas are greatly appreciated.

 

Sounds like your on the right track with the accelerator pump. Also replacing that other line wouldn't be a bad idea.

 

Mine does this until it backfires into the carb most of the time. After the first pop, it seems to run much better. I'm thinking the vac advance is sticky. So that might be somewhere to look.

 

An update:

Wasn't the accelerator pump diaphragm. Checked it this afternoon and looked good, but went ahead an replaced with one I bought a while back.

Starting to lean toward an inoperative vacuum advance. This is a reman distributor a couple of years old which I have already had to replace the magnetic pickup in. No telling what quality of parts were used in the reman.

Any other ideas welcome.

 

interesting, i also have the same problem just like you said, under acceleration. I recently swapped intake manifold, installed a 4bbl carb, fuel filter, lines, gas tank, ignition module, and a lot of other random stuff...and i still have the problem. I think i am also gonna check the vacume advance thing on the distributer as well as that thingy on the pass. side that is some type of vac. resivoar or something? maybe a leak somewhere if i figure it out, i will let you guys know

 

I Have pretty much eliminated the fuel system as the culprit. I have had a couple of folks, who know somewhat more than I do about vehicles, drive/ride with me in the vehicle. Both think it is something cutting out in the ignition system.

Don't think it is the module as it is a Motorcraft module that I bought new several years ago (though anything is possible). It currently has new plugs, wires, cap and rotor. That essentially leaves coil and distributor. The distributor is a reman unit I bought ~3 years ago and which I have already had to replace the pickup in (Echlin pickup now) as the original pickup in the unit failed after about a year. I've got a new HEI distributor that I may slap in there just to see if it cures the problem. Of course, that won't tell me which component was failing.

 

I have the same issue on my truck. I need to start with fuel though because the tanks have never been cleaned and I dont keep much gas in it so probably picks up crap in the tank and clogs the filter. I will change the filter and blow the line out. If that doesnt work then I will start with the distributor. Thats the only thing I havent changed on this truck and no telling how old it is. Please let us know what you find.

 

1967/72 F100/350's with the optional 25 gallon fuel tank / 1973/79 F100/350's w/mid-ship and aft axle fuel tanks.

There is a 3/8" I.D. plastic mesh filter screen (D1AZ-9A011-A) which slides onto the fuel sending unit's pickup tube.

When was the last time it was replaced?

The same filter fits 1965/79 Passenger Cars (except 1971/79 Maverick/Comet/Pinto/Bobcat-it's different: 5/16" I.D.), 1965/79 Econolines and 1966/79 Bronco's.
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When a DuraSpark ignition module begins to fail, the engine dies. Engine heat is the mortal enemy of these modules, especially if radiating from the V8's red hot left exhaust manifold.

The module is mounted on the left fender inner apron. The three step scenario of a failing module:

1) Module overheats, engine dies. Module cools down, engine restarts almost at once.

2) Module overheats, engine dies. Module cools down, engine restarts in an hour...or so.

3) Sooner or later, usually sooner, the module overheats, BURNS OUT. Now the engine will not restart!

Since the module usually fails when it overheats, taking it to a parts store to be tested...it usually tests OK, unless the little darlin' has burned out.

The DuraSpark modules were so unreliable in the 1970's, that Ford techs referred to them as NEVER/Spark.

 

i had the exact symptoms it was my coil got a new one and it runs good now so i almost guarantee its the coil

 

Leaning toward the coil. Need to get my meter out and check it. Just wish Motorcraft still made coils for the application.

 

D5PZ-12029-A .. Ignition Coil (Motorcraft DG-313-A) / Obsolete

All 1974 FoMoCo vehicles with DuraSpark.
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D5AZ-12029-A .. Ignition Coil (Motorcraft DG-314) / Obsolete

All 1975/79 FoMoCo vehicles w/DuraSpark, except some '77/79 vehicles sold new in CA.
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D7AZ-12029-A .. Ignition Coil (Motorcraft DG-316) / Obsolete

1977/79 CA vehicles using the D7AZ-12A199-A module that has the red plastic square.
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Now, which coil would you like? If D5AZ-12029-A .. I found over 40 NOS.

A Motorcraft jobber in Phoenix has 21 (USD = 602-258-1900). Is Arlington considered East TX? Antique Auto Supply has 1 = 817-275-2381.

 

Thanks for your help, Bill. I hope to get the chance to test the coil this evening. Then I should know what I need to do (or keep looking for the problem).

 

You know I am having the same problem. I have already replaced the coil, plugs,wires, cap and rotor and still its doing it. It runs great when in nutural revs smooth but once in gear it starts acting up. it's gotta be the distributor cuz that's the last thing has not been checked. Keep us posted what it is .

 

I had pretty much the same problem and it was due to a bad wire coming from the distributor. I had tried everything else and was really pissed when it was something so simple so I'd check that out before you buy new parts.

 

i know its not the coil causr i also replaced mine but with a accel coil....still have the same problem. Like 74sb4x4 said, mine also revs great in park or nutrall...only acts up in gear while accelerating