DURASPARK IGNITION COIL CONNECTOR PIGTAIL TO HARNESS 76-86 FORD PRODUCTS JEEP AM | eBay

 

I'm not sold on this theory. Maybe on an old points-style distributor, but not with an electronic one.

With points, if the bearings are severely worn, the shaft could bounce around. This would cause the dwell and timing to fluctuate. But with an electronic version, the dwell would remain constant unless the slop was so bad that the reluctor hit the pickup. Or maybe if the reluctor bounced so far away, the pickup couldn't count the pulses. Either way, the pickup is just looking for pulses from the spinning reluctor. Within a broad range, the gap isn't critical.

But IF the wear was bad enough to cause misfiring, such excessive slop would be easy to spot by wiggling the shaft. You'd also see evidence of the reluctor contacting the pickup.

 

You know KR I would have to side with you on that.


Someone said something about the volt signal/ wave from the ALT. a cause? I don't see this happening but as a test could you remove the belt and test.
I don't remember if that was brought up or not and I don't think the drain on the batt. for that short time would be an issue.
Dave ----

 

Here's a good article with more details. It's written for a certain unnamed brand, but the principles are the same for any distributor with centrifugal and vacuum advance:

Everything You Wanted to Know About Vacuum Advance and Ignition Timing

 

Good write-up. However, I beg to differ on the source of vacuum for the advance. It isn't as cut-and-dried as they want to make it. And an automatic transmission seriously complicates things. But otherwise I agree with it. (Not that it matters whether I agree or not, but just so's you know.)

 

That was me. We're 8 pages into this thread with no resolution in sight. I've suggested (at least twice) to check for AC ripple. I'm trying to rule out some of the stranger possibilities, without the expense and hassle of replacing parts that may be perfectly good.

AC ripple is a sneaky problem, as it often has few symptoms other than erratic electronics. And what's the most electronicky (that's a legit word ) component on a carbureted truck of this era? The ignition system!

Testing is quick and easy. Directions were given in a previous post(s). One caveat: A voltmeter isn't really designed to measure AC ripple superimposed on a DC signal. Ideally you need an oscilloscope, which few of us have. Some voltmeters can display a meaningful value for troubleshooting, but not all meters can. To avoid any possibility of having a bad alternator but the meter can't tell, Dave's suggestion to remove the drive belt would be 100% conclusive.

The only other thing I've got is to check the fuel pressure while the fault is occurring.

You can change the distributor if you'd like. I don't think that's it, but maybe I'm less correct than usual this time.

After that, about the only recourse is obtaining a platinum credit card and leaving it with your local parts store as you change every part under the hood one at a time. Some stores offer a parts catapult service. You give them the coordinates where the truck is parked with the hood open. The friendly clerk loads the catapult and fires the parts in the general direction of the problem. This generally doesn't fix anything, but I have a few coworkers that prefer this method of troubleshooting.

 

Gary, it's on the internet. It must be true...

One other thought. Maybe there's a broken or weak valve spring (or similar breathing issue) that only acts up under certain RPM and vacuum combinations.

This is a slow test, but will find it:

Dynamic Compression Test

Other variations can be found by searching for "dynamic compression test" or "cranking compression test".

 

With the ALT and AC wave it got me of thinking of EMF but a vary long shot!
What would cause it to happen at a given RPM under no load and different RPM under load. Not much that I can think of.


Now with everything done and tested and you now bringing up weak or broken valve spring(s). I have heard of motors (race) not reaching a high RPM that it should and back firing.
On a street motor why cant it show up as what Whisler has happening?
Unless he was to see a broken spring the only way I know of to check for weak springs is to remove them for testing.


Me I would do a fuel PSI test and pop the ALT belt off to see if AC wave is the cause then look for broken valve springs by just pulling the valve cover.
If that all looks good I would just drive the crap out of it till something big happens and needs looking at.
Dave ----

 

Has the distributor pickup been changed yet? I was thinking it had been, but now can't find any mention of it. I'd still think a pickup fault would make the tach erratic, but who knows?

Since we're just guessing at this point, that's as good a stab as anything else.

 

Thanks for all the suggestions, guys. I will try the "remove the alt. belt" idea and try to figure out a set-up to check fuel pressure while driving.
In the meantime I am going to go completely thru the carb. again. If nothing shows up, I will pull the distributor and check for something really weird in there (it is a re-man. after all). I would like to recurve it anyway and that would be a good opportunity to become very familiar with distributor internals.
Thanks again.

 

A couple of thoughts for a test setup. Do you still have the metal line between the fuel pump and the filter at the carb inlet?

1) You could remove the filter and make a tee with the correct fittings to replace it. You'd be fine to run briefly without a filter while troubleshooting.

2) Disconnect the metal line at both ends and run a temporary piece of flex hose instead. Carefully secure the hose and keep it away from heat and moving parts. It would be easy enough to tee into a piece of hose.

 

Kr98664: Thanks for the pressure test set-up suggestions and to answer your question the pickup has not been changed. However it does check out resistance wise at 587 Ohms. (400-700 ohms is good range.)

 

Just had a remembrance. At the Okie GTG a couple of years ago one gentleman from Kansas (we also had people from Alabama and Colorado) was having trouble with his vacuum advance. If he connected it the engine died. Paul diagnosed that immediately - bad wires in the pickup. Sure enough, the insulation was gone at one point and when the vacuum advance came on fully the pickup grounded to the case.

So, could the wires on yours be grounding just briefly? And at only one range of vacuum?

 

I thought Whisler installed a reman dist.?
If so I don't think it would have bad wires or even a bad pickup.
Could they be bad? Why yes but not likely.


Easy enough to check the wires, disconnect the vacuum advance hose and test.
Dave ----

 

Exactly! (10 characters! )