If it happens in the driveway, meaning with no load on the engine, you can probably rule out the secondary side of the ignition system, meaning the coil, cap, rotor, plug wires, and plugs. So, I am now thinking the pickup in the distributor is the problem. But, I will quickly admit I'm grasping at straws as that is not the typical failure mode for a pickup, as it usually quits when warm and then works when it cools.

 

Back to post #1 here, as I fire up my electric headscratcher:

You mentioned the problem changed after replacing the plugs, cap, and rotor. What if one of those parts was marginally defective? Something changed, right? I'd be willing to gamble your hard earned money on another cap and rotor, since we're to the point of loading the parts catapult. Get a different brand as a precaution, in case of a defective batch.

Has your FrankenTruck ever run okay with this ignition system? In other words, how long have you had this system and when did the problem start?

 

Gary: It does stumble with no load but at 1000 RPM higher The distributor is a reman. Cardone that probably has less than 5K miles on it. How does one test the pickup coil on this? Looking at the inside of the distributor, removal procedure is not immediately apparent.

I think I will try holding second gear (to stay below speed limit) and see what I get at higher RPM on level ground. I also have a longer straight hill nearby that I know causes the issue. I may re-install the old coil(to keep from burning up the new one) and the jumper and take a run at that hill. I really don't want to ruin the ignition module with high voltage either but I want this problem solved.

Wild idea, could my main jets still be too small? Doesn't seem likely since I should just be into the power valve but what do i know?

Any other suggestions?

 

At this point, I don't think the plugs, etc. did anything but push the problem to slightly higher RPM. I think my erroneous conclusion came from running out of hill (short steep hill with curve at the top) before the problem reared its ugly head again.

This system has been in service for about 9 month. Didn't really do too many hard pulls in that time but I did notice occasional stumbles when i did. The problem became apparent when I was testing main jet/power valve combo to see if I was running lean. I don't think it has ever been quite right.

When the engine was in the other truck I didn't drive it hard due to rusty frame, the reason for the swap. The engine sitting for 12-15 months while we were getting the '89 truck ready probably didn't help anything either. My buddy whose shop and equipment we were using had a serious health problem right in the middle of things which really delayed things. The fact that he lives 50 miles away didn't help speed things along either.

 

I don't think it is the jetting. As you said, you should be into the power valve and that makes the air/fuel ratio quite rich. If it wasn't doing this in the driveway I'd say it might be lack of fuel to the carb, but you don't use much fuel when not in gear, so I don't think it is that either.

That means it is probably ignition. But, with no load on the engine you aren't taxing the system. However, if you don't have enough voltage to the coil it cannot create enough spark to jump across the high cylinder pressure when under load. And, it makes sense that it would also do it at higher RPM without the load. So, I'm still thinking it has something to do with the resistor. Given that, swap coils again and jumper the resistor.

 

Is the choke pulling closed at max CFM?

Wires are about the only thing left.

Make sure the wires are well separated and don't run touching parallel to one and other.

Also, are you running a big sized distributor cap or a small one? You need the larger sized distributor cap to prevent spark scatter.

The high voltage dura spark is too hot for the small distributor cap. The terminals are too close together.

Post some photos of your motor.

 

Would I have a problem checking voltage at the coil with the engine running? that way I would now the actual voltage applied to the coil.

Brnfree: wires are well separated and I do have the large distributor cap. I do have one plug boot that is pretty close to a header pipe, not touching but closer than I would like. I think I will try to get a heat shield type sleeve for over that boot. Still have to check the wires or maybe just replace them.

 

You can check the + side of the coil with the motor running and it should read a couple volts below battery voltage.

 

Most meters will not report an accurate voltage on the primary side of the coil while running. In fact, I doubt any meter will. The on/off gives them problems, and the back EMF that goes well above battery voltage confuses them completely.

 

Ok being you replaced the dist. I don't think the pickup would be bad. BTW Gary I was thinking pickup too.

if you question the plug wires I would just replace them for piece of mind.

what make & number plugs you running? I ask because you said the issue moved up in RPM changing them and cap,rotor.
Dave - - - -

 

"Is the choke pulling closed at max CFM?"
Brnfree: Just noticed this part of your question. I don't even know how I would check that. However the choke seems to work very well. Frankenstein starts pretty well even on cold mornings; push the pedal to the floor twice and hit the key. It is opening fully when warmed up.

FuzzFace2: when I did the DS2 conversion on the '84 F150 (now departed) I picked up the coil, harness, module and carb. from a well respected supplier of used Ford truck parts in Northeast OK.. He wisely advised that his 30 year old distributor was probably not a good investment and suggested new or reman. Money being a little tight at the time I went with reman.

The plugs I just installed are Autolite 25 and they replaced Motorcraft platinum that had probably been in for about 20K miles. When we swapped engines my buddy sandblasted and re-gapped them, which I now assume is not advised since even filing platinum plugs is a no-no.

Somehow the basic problem has still not been corrected because changes either seem to have no effect or move the problem to a slightly higher RPM.

I really appreciate all the help you guys have given. I'm not very skilled at diagnosing electrical problems. Most of this type of issue that I have previously encountered have been addressed by replacing the usual suspects, plugs rotor, cap, etc. The only other really weird electrical issue that I've encountered was with a 1964 Ford wagon that would cut out on hard turns but only in one direction. I don't remember how I came to suspect the coil, but I had another one and when I swapped it in the problem went away. Diagnosis by luck I guess.

Anyway thanks again to all and if another other ideas come to mind let me know.

 

How about this? Even though different meters will give different readings, what if OP listed what type of meter (brand and model number) was used and exactly where the leads were connected? With any luck, somebody on the forum will have the same meter and a truck with DuraSpark ignition in good working order. Compare the readings and see if they are close. (Meanwhile, I'll be patting myself on the back. If I get tired, is anybody willing to take over? I'm okay if you guys want to split it up into shifts.)

Other than that, we're probably getting into Twilight Zone stuff for a fix. Kind of a long shot, but what about AC interference from a defective alternator? This can cause all sorts of strange behavior, and is super easy to check.

Check for excessive AC ripple right at the alternator, not at the battery. The battery acts as a giant capacitor to smooth out the normal AC ripple. But if a diode has failed, the battery can mask it to some extent, so it's important to get the reading as close to the source as possible.

Put your meter's (+) lead on the big insulated terminal on the back of the alternator. Put the (-) lead on the alternator case. Set your meter to AC volts, not DC. Start the engine and run it at idle and approximately 2000 RPM, with minimal electrical load. Repeat with a heavy electrical load, such as heater fan and high beams on. If any reading is over 0.5 VAC, your alternator has a bad diode.
​​​​​
Another quick check (not as accurate) is to increase the electrical load when the truck is acting up. Set the heater fan to HI and turn on the high beams. In some cases, the extra load dampens the AC spikes. However, in other cases, the extra demand makes the alternator act up even more. Either way, if the rough running suddenly changes, whether better or worse, you've uncovered a big clue.

One last thought. Nobody really knows if this is a fuel or ignition problem. We can kid ourselves, but mostly all anybody can do is make educated guesses. You mentioned having checked the fuel pump output. This was volume and pressure at cranking speed with the line disconnected, right? I suggested teeing in a pressure gauge so you can see what's happening with the fuel pressure when the engine acts up. Another long shot, for sure, but what if the pump only acts up at high engine RPM? Maybe the return spring is weak holding the arm against the cam eccentric. If so, the pump might work fine at low RPM yet tend to float at higher speeds. Just a thought...

 

KR - Your idea on the DVM should work, although finding someone with the same meter may not be easy. The best way is with an oscilloscope, and I already have a set of good traces.

And, you are right about the rest as well. We are making guesses, and all of us come with varying experience in these things, so there's no way to know who may be right. That's what I like about the collaboration here.

 

Tangent Alert!

New oscilloscope here, post #27:

https://www.ford-trucks.com/forums/1...our-dvoms.html

 

If you think it is an issue from the ALT just remove the belt and give it a try.
This way the ALT is not putting anything out to cause anything.


What is the spark plug gap? If not right (too wide) it will make the IGN. system work harder and can cause issues.
I think something with them because when you changed them it made a change.
Dave ----