I'm working on a 79 F-250 with a 390 in it. I've got a problem with it bucking under a light load. It runs great at idle and at full throttle, with plenty of power. I replaced the points distributor with a Duraspark unit, which helped immensly, but it's still not right. I've tried a known good carb and coil also, with no change.

Also, I'm only getting 6-7 volts at the positive side of the coil, while my 79 which hasn't been cobbled together like this one, has around 11. If this was part of the problem why would it run so good wide open? 6 volts is correct for points though, right?

Could a bad plug do this? They are fairly new, but whoever installed them may have dropped one, I don't know.

Thanks guys for any help you can give me. I've never been around many FE's, but this seems like it'll run pretty good, for being stock with a 2 barrel.

 

The harness module connector with the red wire should have battery voltage in the run position. the green lead on the module should have the same in run. Is there a resistor in the circuit somewhere??
WOT-don't know
Any failure or weakness in the secondary ignition can cause a misfire, I would inspect the wires, plugs, cap, and rotor for problems.

 

Sounds Like to me a High Voltage break down In the Ignition system somewhere??

Russ

 

It's got new plugs, wires, cap, and rotor. I think I will try a new set of plugs though. The more I think about it, it acts similar to the way my 95 F-150 did when it had a bad plug.

 

Ignition Coil.

 

Brown 4 X 4 stated he installed a known good carb and coil, same problem. Did you meter your ignition wires? Plugs can play with your mind. I have a 50 year old Champion Plug Tester, you install a plug, add air pressure (120-150 psi) power up the model T coil and check the plug spark under pressure. Many of plugs have failed the pressure check. Brown are you sure the vacuum advance is working, diaphragm broken, full travel and returning? Connected to ported on the carb?
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Stumbling not bucking due to running retarded, no vacuum advance, forgot to add in last reply. Careful as you can blow out your mufflers doing road tests.
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Does it have an EGR? part-throttle, light load is when the EGR is supposed to open. Just curious...

The voltage at the coil does sound low - if it had points, there is a resistor in the "run" line. If the Duraspark was connected to that, that could be a big part (or all) of the problem. In '73 to '76, the Ford "breakerless" ignition used a resistor. Duraspark I('77) did not (according to Motor's schematic), but Duraspark II did - if it's a '79, I think it had the Duraspark II, so there is a resistor in there somewhere.

Where did the Duraspark come from? What dizzy? etc. etc. ad inifinitum

 

Ignition breakdown would be the greatest at full throttle and he stated it runs good at idle and full throttle. Did you check the vacuum diaphragm for leak and travel plus is the mechanical lubed and functioning correctly thru it's rpm range?
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I once had a vacuum advance that moved the point plate to where the points wire lead shorted out(abraded bare wire). It ran at idle and WOT(vac adv not moving at WOT) but missed real bad under light load, part throttle.

 

Thank you Putt as I did that myself, ran good for 20 miles then WTF!
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Well you guys that mentioned vacuum advance are partly right. After putting a new set of plugs in with no change, I got to thinking the problem only occurs when the vacuum advance kicks in. I unhooked the vacuum advance, and it runs great. Now my question is why? Something isn't right to make it do this. It's not in the distributor, as it did it with the old points distributor. The Duraspark one is a remanufactured distributor from Carquest. The module is one from a junkyard (Motorcraft) tested on my truck.

Krewat, no EGR on this engine.

BTW, according to my '79 manual, the difference between Duraspark II and I (called SS 1) is the larger cap, rotor, wires, and spark plug gap. The resistor is 1.35 ohms versus 1.10 ohms for DS II. The coil, module, and distributor are all the same. My book doesn't mention points, so I don't know what the coil voltage would have been for them.

 

It sounds like your vacuum advance can is shot. It can fail so full advance occurs with any vacuum signal-too much advance=pre-ignition, bucking. Maybe anyways

 

How can it fail to full advance? There is a spring that holds it in the zero advance position. Maybe a dual dyaphram could, but also not likely.

 

I think he's thinking about Chevies

If the vacuum advance is leaking, during the peak vacuum advance, the carb is not giving the right mix.

Stick a tube on the vacuum advance pot, and suck on it with the cap off - if the plate moves a little or not at all, and you can suck on the tube and not come up with vacuum, it's shot.