Hey guys, appreciate your responses to this. I'm quite annoyed with this problem, as I've just about replaced everything. Let me give you the background.

1981 F150, inline six. Truck has lots of miles on it, but the engine still has good compression. A few months back, it started running rough. The plugs seemed fouled, so I bought new motorcraft replacements and gapped them to spec. It ran great for about an hour, then back to missing. I assumed it was the carb, so I got another and rebuilt it. No change. I then replaced plug wires, cap and rotor. No change. I then put in another distributor that I had laying around. Still no change. I also changed the duraspark unit, and coil - and I also re-wired from the duraspark to the distributor and sensors. I finally put the timing light on it to make sure it was within a good range. I noticed that the #1 plug wasn't firing right, just intermittently and very random. So I put it on the other wires - same thing. I pulled the plug wire from #1, and verified that the timing light was correct. It is. Then I pulled the wire, and set it so there was a gap between the wire and the top of the plug. Amazingly it fires perfect like this - all 6 plugs, same thing. In fact, #4 won't fire at all with the wire on the plug, only with a gap between the wire and plug. I cleaned up all the plugs and changed the gap on them to be wider, and now they fire as they are supposed to, with one exception. If I put the timing light on ANY wire and hit the accelerator, for a brief second they don't fire. I'm stumped on this. I bought some new plugs today, even though these are only a month old, and less than a couple hours of run time on them.

Anyone have a clue as to what might be going on? I've heard that in a system with a weak spark, high compression and a rich gas mixture can cause a plug not to spark... but still, that means making the gap smaller should help, not larger....? I'm stumped - hope someone can lead me in the right direction.

 

What is the voltage on the battery when it's running? What's the voltage on the coil + while it's running? When you check the coil, ground the neg meter lead on the engine block.

 

I'll check the things as you asked. When diagnosing stuff, I did check the alternator, it's putting out around 14 volts. I checked the positive side of the coil before I messed with the wiring and it seems like it was right around 10 volts. I'll check it again as you mentioned, along with the battery voltage.

 

Those voltages sound ok. Is this a truck you are just messing with around in the yard, or do you drive it on the road? If you drive it on the road, does it clear up after awhile?

If this is just a project that you start and run around in the yard, then the engine and the plugs may not be getting hot enough and are fouling all the time. Also with project vehicles like that, they have a problem with the oil getting thin from fuel wash down in the cylinders.

Another thought. What sparkplugs are you running? What are the exact numbers of the plugs?

 

Try running a jumper wire, battery + to coil + ... which will put full voltage at the coil for a short test to see if it helps give you get a stead spark.

Check and clean the coil NEG ground point which is inside the DISSY under a clamping screw. Make sure the DISSY is well grounded, add a GRD jumper wire for testing if your not sure.

The current flow is, in the coil +, out the coil -, to the DS2 module on the fender, back to the DISSY to ground screw under the cap. There should not be resistance in the ground path after the coil. If there is it will cause a voltage drop. All of the voltage applied to the coil should be dropping across the coil and not after it in a high resiatance conection in the ground path.

The black wire is the ground wire going to the Dissy, you can also add an addition ground to this wire for testing, the other 2 wires are for the MAG pickup in the Dissy.

Making the SPARK jump a bigger gap is forcing a higher voltage output from the coil.

PS... this is a hard thing to troubleshoot

 

Franklin, it is used mainly to go 4 wheeling, but I drive it around daily at times too. During this period of running bad, I was actually driving it down the highway when it started missing.

Jim, I did try jumping battery voltage to the coil when I originally found the problem, and nothing changed. The black wire to the dizzy comes from the DS2 module, right? Where is it grounded? The module itself is mounted onto the plastic wheel well, so it isn't through the case. I need to chase down the grounds and make sure everything is tight.

I'll report back when I get a chance to look things over again - probably on Thursday evening. Thanks again guys.

 

the only GRD is under the Dissy cap, were the harness changes from 3 wires to 2 wires, it is the weak link in the system, You can change or add a NEW GRD any where after/downsream the DS2 module using the black wire.

 

Well guys, don't know what to tell ya. 14 volts at battery, 10.8 at the coil, varying up to 11.2 when revving. Ground is all good in dizzy. With the 1 month old plugs, still had the weird no spark issue when quickly hitting the accelerator. So, last hope was to replace the plugs. It fixed the problem so far.

Is there anyway that a weak spark over a short period of time can ruin the plugs?? I cleaned them up nicely and it still had the missing spark on those, but these new plugs don't do it.

I'm hoping that bad wiring and stuff just caused a weak spark. After I got the new parts, for some reason those plugs were just too bad to work? No clue, I thought a spark plug either sparked or it didn't.....

 

I have had plugs foul, and sometimes cannot get them to fire after that no matter what I do to them. I don't know what it is. Sometimes I will get one or two to foul, and it takes 5 or 6 miles of hard interstate driving before they finally start to clear up, if they ever do.