I am stumped. 1973 360 with ford distributor and pertronix ignition. Curently truck is running on only 3 cylinders. Spark only on numbers 4,5, and 8 reliably. Last Friday it started to miss a bit. Gone down hill ever since. I have replaced the following. The Pertronix ignition with new. New plugs cap rotor and wires. No improvement. New coil. No improvement. I have reinstalled all of the original pieces one at at time to see if new was faulty. No improvement. No spark at plugs. Backtrack to wires and no spark thru wires. Check for spark at cap and only have spark on the above cylinders. All the others are dead.

I have no idea why three cylinders are getting spark and the other 5 aren't even getting out of the distributor cap. I have tried to wobble dist. shaft and it doesn't.

Compression is on low side, but that shouldn't affect spark.

Can't give ya an accurate timing since number one isn't firing.

Any and all comments, suggestions, and ideas are encouraged!

 

What type of Pertronix unit? The little module that goes inside the dist in place of points,, or a stand alone box outside the dist?

If it's the one that goes inside, check the pertronix plastic collar that slips down. The bottom has been known to come loose and the magnets come out.

Also, those little units need to be grounded. If you ground it to the dist case, is the dist grounded to the engine?

Some of this sounds a little off the wall, but as you've changed everything, teh more common reasons don't make sense (like plug wires.....you replaced them, and no change)

My best guess is your +12 VDC to the Pertronix unit. If it's wired through the firewall mounted solenoid, you may have trouble there. I connected mine to the coil (+) which means it only gets 12VDC from that solenoid while cranking. After that, it gets it from the ballast resistor. Luckily I don't have a ballast resistor as it's built inot my coil. But if you do, and that's where your Pertronix is getting power from, it could have been on the edge of working since it was installed, and with a bit of ballast resistor degridation (particualrly at the connections) the voltage may be no longer be sufficient.

I'd also check fuses as they can corrode in the fuse box and cut the voltage down. Even the ignition switch is suspect. Same with any firewall wire harness plugs. All easy to check, ..start at the igntion unit and make sure you have 12VDC (actually 13 to 14 with engine running). At the same time, check the coil (+) post and make sure you have about 9 volts if a ballast resistor is installed.

 

In terms of checking timing, you can check it with the plug that's opposite #1 on the distributor cap if that one is firing.

Otherwise, something is very odd. You changed the pertronix, but is the wheel on the shaft clean and straight?

 

Yes it is the module that goes into the dist. I have taken it out and installed a new unit. That didn't help. The collar on the shaft is tight, both the existing one and the new one have fit tight.

Something is definetly wonky! I will check the mounting of the module in the dist. this evening.

Summary

new plugs
new wires
new cap
new rotor
new petronix module
new coil

Only heave spark out of dist. for three plugs. The other five are dead. I mean I can stick a screwdriver in to the plug wire hole on cap and short across and get nothing. The three that do work will short damn near 3/4 of an inch!

HELP

 

So it definately seems to be individual cylinders vs an overall problem, so power and grounds or anything common to the system probably isn't the problem (although not quite ruled out).

And the problem has been found back as far as the dist cap output, which eliminates wires and plugs

Here's how I would further trace it:

Remove the dist cap. Take the coil wire out of the dist cap and connect a sparkplug to the free end. Ground the plug. So you have a sparkplug hooked to the coil.

Now you can do a couple tests:

1/ have someone crank the engine over and see if the plug fires regularly or not (if it does, then cap and rotor button, even though you replaced them)

2/ With ignition off, you can rotate the engine 90 degrees at a time with a wrench. Use damper points 20Deg, 110, 200, 290, 380(which is 20 again), and so on through the second revolution. At each point, stop rotating the engine, and rotate the rotor button by hand (spark advance) and you should get a spark. If you don't, maybe you can see what's going on as the dist rotates and the cap is off.

Firing order is 15426378 , which can be re-written as 81542637, and you have spark on almost the first 1/2, i.e. 8, 5 and 4. Only 1 is missing. So it looks like 1/2 of the dist shaft is working, and the other 1/2 is not. Like the dist shaft was bent, or the dist cap canted.

With all the parts you changed, I am stumped. But I would try the test with cap off, and a single sparkplug connected to the coil output. You should be able to further identify enough of the problem to figure out what's happening.

If all else fails, pull the dist, chek for a slipping drive gear, broken split pin, cracked or broken dist shaft, etc.

 

Thanks for the info.

I have hooked a timing light to the coil wire and it fires rapidly and evenly. I am believeing that means I have good spark to the dist.

I agree with the idea that half is working and half isn't. I have checked and rechecked the fit of both dist. caps. And I will do so again tonight.

Thanks for the advice. Don't stop now.

 

Sure sounds like cap or rotor now. I know you put it new ones. Was it raining, or di the new ones somehow get wet?

Look inside the cap for black thin lines from the terminals (carbon tracking). Chekc the old one too.

I once had a problem of an engine that wouldn't start, yet it was getting spark and fuel. Turned out the rotor button had carbon tracked on the underside of the plastic counterweight opposite the brass contact. So it was firing 180 Deg out. Yours could be arcing to ground instead, so no spark. Mind you, yours has some good sparks, so I would concentrate on the dist cap, maybe even try the old one. If you find carbon tracking, you can get a temp fix by grinding the carbon off. Wipe down the inside with alcohol to remove moisture as well.

 

TaaDaa! Its fixed! Moisture was the culprit. I took the cap off and pulled the petronix module out and sprayed electrical contact cleaner on breaker plate and also in the cap. Reinstalled and...... Runs like a champ!!! Not sure if it was not getting a good ground thru the mounting of the module or, more likely, moisture was interfering with the cap. Its odd that both old and new caps did the exact same thing. However, I don't care. The old girl is up and running again and I can now get the load of garbage on her taken to the dump.

Thanks to both of you for your responses and advice.