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I've been hot on the trail of an irritating misfire on my '84 351W. It has the Duraspark II ignition system. Lately I've been getting a pronounced stumble between 1700-2000 RPM. As soon as I pass 2000 RPM, the misfire disappears, as if a switch has been flipped. More than a cylinder not firing, it's seems as if one (or more) is firing at the wrong time, as if plug wires were crossed or I was getting an inductance misfire between adjacent plug wires. When the fault is active, the steering wheel would shake, it was that strong. Nothing new has been done to the engine when all this started, basically out of the blue.
One thing I did notice was an insufficient gap between the rotor and stator inside the distributor. You can see a wear mark on the face of the stator (pickup coil) in this picture:
Of the 8 teeth on the rotor, 4 of them have shiny marks.
Is there any sort of adjustment possible to move the stator further away from the rotor? If so, what is the specification? I didn't see anything in the manual. There seems to be only normal play inside the distributor, nothing excessive compared to two other used units I have. Neither of those spares show any evidence of the rotor hitting the stator.
Is this the problem causing the rough running? I don't know, but I'm following my philosophy to fix known faults first before digging deeper elsewhere. I figure it can't be good for these parts to be making contact.
As an aside, I'd like to also mention my philosophy to avoid making too many changes at once while troubleshooting. The distributor cap and rotor looked a little long in the tooth, so I replaced them. I was immediately greeted with much worse stumbling at all speeds, and even some backfiring. After some serious head-scratching and professional-grade foul language, I reviewed my recent work. I reinstalled the old cap and rotor and immediately cleared up this new problem. Brand new premium grade parts from NAPA, too, bad right out of the box.
While looking over the pickup coil I remember there being a post someone also had a miss fire and when he pulled the coil he found the magnet was cracked.
Could that be why they are "kissing"?
Dave ----
Do you mean the rotor? That might explain why it’s only hitting on four of the eight teeth.
Not the rotor and maybe magnet was the wrong description but the coil part of the pickup that you are showing the marks on.
If it is cracked and shifted it could be why it hits?
Dave ----
I have ordered a Cardone reman distributor. Supposedly I can pick it up this afternoon. I had considered replacing just the pickup coil, but am not sure if worn bushings may have contributed to the problem. It may be a few days before I can install it, but will update then.
I forgot to mention I'm pretty sure this problem is in the ignition system, as it seems to be strictly RPM based. Even when the engine has just been started cold, the problem is there. If this was a carb issue, I'd think it would change (better or worse) when the choke is on. Acceleration or steady-state doesn't matter, either. Only if coasting or compression braking does the problem go away in that RPM range. Compression? Possibly, but I'll investigate that later if fixing this obvious interference doesn't help.
Been awhile since I had one apart. Are the holes slotted? I feel like they have a little wiggle room if they aren't. Have you tried loosening the screws and seeing if you have enough play to adjust it? Another thing I'd consider is the advance springs. That may be right where the heavy spring starts to come in and it's moving in and out a bit at that rpm range. Last thing I'd look at is if there is excessive play in the shaft. I ended up replacing mine because it had too much play.
I like Dave's theory that the pickup coil (blue in the picture) has cracked and expanded, closing the gap with the rotor. I will know more when I work on it in a few days.
I hope thats what the cause is.
I just remember a member having issues I think with a miss fire like you are having and found the coil had a crack in it.
Once replaced all was good.
I just thought of something.
1 of the ways to check for a stretched timing chain and play in the dist. bushings when they had points in them was to use a timing light.
When the bushing went bad the shaft would move around changing the point gap and that changed the timing.
Then to make sure what the cause was, chain or dist. you would use a dwell meter and if the dwell bounces its the dist. Yep I am that old LOL
I dont know if that holds true with electronic dist.? Will a Dwell meter even work on a electronic dist.?
Dave ----
I like Dave's theory that the pickup coil (blue in the picture) has cracked and expanded, closing the gap with the rotor. I will know more when I work on it in a few days.
My eyes must be getting worse, looked like hex screws in the photo.
1 of the ways to check for a stretched timing chain and play in the dist. bushings when they had points in them was to use a timing light.
When the bushing went bad the shaft would move around changing the point gap and that changed the timing.
Then to make sure what the cause was, chain or dist. you would use a dwell meter and if the dwell bounces its the dist. Yep I am that old LOL
I dont know if that holds true with electronic dist.? Will a Dwell meter even work on a electronic dist.?
A dwell meter does work on an electronic distributor, but the dwell itself is not adjustable.
With either style of distributor, dwell refers to how long (degrees of rotation) the coil's primary circuit is energized.
With points, dwell refers to when the contacts are closed. If you change the gap, and thus the amount of time the primary circuit is energized, the dwell will change. The start depends on when the points close. The end depends on when the points open.
With an electronic distributor, dwell is controlled by the ignition module. As the rotor spins past the pickup coil, this signals the module to energize the primary circuit. The rotor action only says when to start the process. The end of the dwell cycle is decided by the module, not the action of the rotor.
So yeah, a dwell meter will work on an electronic distributor. Unfortunately, it can't tell you anything about play in the shaft like it could with points.
It was just a thought is all.
With points pushing on 1 side of the shaft it was some what common for the top bushings to wear and why the play and point changing gap.
Dave ----
So did you old distributor have play in the shaft so the star was touching the coil or the coil moved??
I plan to do a post-mortem on the bad distributor. Play is a little sloppier than the new (reman) unit I just installed, but not much. It's almost as if the rotor is not concentric or the shaft is bent. Curiously, the centrifugal advance seems to work freely, so it's not like that part has failed and caused the rotor to be out of alignment with the main shaft. Where the one side of the rotor is hitting, there's basically no clearance and it can't be pulled back. It's not as if the rotor was wobbling around *****-nilly and hitting the pickup coil randomly. It is being forced over, but only on 4 of the rotor teeth. Two adjacent teeth were hitting firmly. The teeth on either side of them were just grazing. The other four on the opposite side were not making contact at all.
I've got some other pressing things around the house, so unfortunately it may be a little while.
