Ladies and Gentlemen, I have an interesting problem. 1978 F250, 460 w/ edelbrock intake and edelbrock 750 cfm carburetor, C6 trans. Ran great until I recently lost spark after getting into town, trouble shooting led me to the distributor pickup coil. No problem, got one in the glovebox, Standard Product. Switch that rascal out, truck fires up, gets me home but runs poorly, spitting back thru carb, popping thru exhaust. Thinking I did something dumb, like disconnect something while I was fiddling with the distributor, I looked everything over, all looked good, which led to getting out the voltmeter and down the rabbit hole of checking everything remotely having anything to do with ignition, and /or engine electrical. Couldn't find anything amiss. Somehow during all this brain dancing around under the hood, I decided to check the timing, pop off the vacuum line to the distributor, cap off drivers side carburetor port and check timing. Runs great, timing on the money, revs good, no popping or spitting. Road test goes well. Plug the distributor vacuum back in, pop and spit. Replaced the vacuum advance module on the distributor, still no good. It only runs well with the distributor vacuum line off the carburetor, and the port capped. What in the world have I done?

 

Stock Ford Duraspark distributor?

 

yes, all stock except for the intake and carburetor

 

Sometimes trying to fix a distributor just isn't worth the hassle... Waaay back in 2007 I need to repair the Duraspark distributor in my '89 5.8. I just knew I'd break off the screws because they were so corroded (Cleveland road salt kills things fast)

A new/reman distributor for your 460 from Autozone is ~$82 (was only $51 back then). Comes with new cap/rotor, pickup module, vacuum advance,.... https://www.autozone.com/external-en...xt=Distributor

 

Your stator is still bolted to a movable breaker plate with a bushing that wears out
Just like points bouncing on a worn breaker plate, your stator can do the same thing
I would be checking the rate of timing advance with a digital timing light
Back when your truck was new, we had trouble with some of the vacuum advances
Some had a weak spring and needed a washer installed under the screw off cap
If your vacuum advance has a screw off nipple, try putting a small washer in there to tighten up the spring
Also, they are adjustable with an allen wrench through the nipple hole
Yours, if new, may not have those provisions
When replacing the stator, you put the timing slot back where it was for the mechanical advance?
That top piece goes on there two ways, where the springs are
One slot allows for more timing advance than the other slot
Your module may have gone haywire if you see no trouble with any of the above
Some of those later modules had 3 connectors, try disconnecting the 3rd set of wires and see what it does
Some of the later ones had a Bmap switch (Versalles and vans) IIRR and California emissions
Unplug that too (if equipped) and see how it runs

 

Manicmechanic007, I took apart the distributor down to the advance springs and weights, everything moves freely, no binding, nothing looks broken. I checked the ignition coil mounting plate, made sure it was flat and installed correctly. Changed out the distributor coil for another new (it wouldn't run at all, nor with the 2 junkyard pickups I had laying around) put the original back in, no go, still the same problem. I crossed the part number I found on the original pickup coil, It led me back to all the replacements I've been using. Maybe its time for a new distributor?

 

Hard? to find a Ford Rotunda Breakerless Ignition Tester these days, but I found a working one on Ebay for 40 bucks
That tester plugs into the wires of your module and determines the reason of the no spark
I bought it because I run a Duraspark on my Boss 302 and after working for Ford all those years, I know the easiest (best) way to test them is the Ford tester
If you lived close, I'd bring mine over
Buried in my garage where my breakout box is somewhere
Generally, only Ford dealers have that tester, they get old and useless they figure, and let their techs have them
Then the techs sell them sometimes, that's also how I acquired a super star2
Course, I do this for a living too, so time is money

 

Usually this problem is caused by the pickup coil wires getting bad and when they are flexed by the moving vacuum advance it causes a misfire but you've eliminated that problem. I've also seen a pickup coil with the wrong polarity cause this problem on an Oldsmobile with an HEI but that seems unlikely on a Ford because all the distributors rotate in the same direction. What happens is that it puts the distributor phasing between cap, rotor and impulse from the pickup off enough that the spark can jump to the wrong terminal. When the vacuum advance moves the pickup this changes the phasing relationship for the worse enough that it misfires.

If you hook up your timing light and check the timing with and without the can connected how much timing is the vacuum advance adding when it pulls up the timing? I've seen those where they just simply advance too far and cause the engine to misfire. Most of them are adjustable using either a small screwdriver or allen wrench inside of the vacuum nipple. Try taking some turns out and see if it fixes the problem. Why this would get out of adjustment who knows. I've seen that on many aftermarket distributors where the can will pull in some outrageous amount of timing, more than 15 degrees crank + the timing brought in by the centrifugal advance.

 

Thanks for that idea. I timed it with the distributor vacuum unconnected, vacuum port on the passenger side of the carb capped. Raising the rpm to (I'm guessing) 2000 rpm got me about 6-8 more degrees btdc over the intial 8 degrees. When I hooked the vacuum line back up and tried it, it popped thru the exhaust and spit back thru the carb making it hard to tell, but looks like no change or little change from just checking the mechanical advance. Then I replaced the distributor vacuum advance module, I adjusted it all the way in, then all the way out, checking it at each full turn, it didn't seem to make any difference, with the vacuum advance hooked up, it would not run well, still popping and spitting. Guys i really appreciate the help. I've never encountered this in my 45 years of hot rodding and fooling around with old vehicles.

 

Went with a new distributor, HEI style, we'll see how long it lasts. Truck is running well at this time.