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Very random misfire....hardly ever happens, but....

1K views 7 replies 3 participants last post by  Argess 
#1 ·
I had a slight misfire the other day. Not a bang from crossfire, but more of a spitting sound from a single no spark situation. I'm a bit premature in posting this as I havn't done anything to check things out yet, but I have a question. Could prolonged idling around town cause enough carbon to build up that when I gave lots of throttle, some carbon got knocked loose and fouled a plug for a single misfire? That might explain why it hasn't happened since. I suppose it could be fuel puddling on the intake manifold floor ...not sure about that one.

I rebuilt my Ford 428 lst year and spent the summer fine tuning the carbs and timing. During that time I had some serious igntiion problems which I thought were over.

I had changed plugs, wires, etc. Major problem was traced to a bad plug, killed by crossfire (old wires). I put another one in.....a good plug with hardly any use, but not new. RF9YCs seem to be hard to get vs the more common RF11YC and I had exhausted local stock, so used one of my used "good one's". It was "mostly" fine for the remainder of last summer....maybe the odd little misfire, but very rare.

I usually drive the 5 miles to my nearest town at reduced rpms to warm the engine (and oil) up and the other day there was a lot of traffic around town so there was a lot of idling and just off-idle driving after getting there. Upon leaving town, a quick shot of the throttle produced great acceleration, but I had this one misfire. Just the normal "spitting sound", not the bang that crossfire can produce.

I havn't had a misfire since then, so although I will pull all the plugs, I don't expect to see anything out of the ordinary.

So, even though I thought I fixed everything, I have lost faith and don't feel like pushing the engine too hard until I fix whatever's wrong. But it's difficult because it hardly ever does it. If it were consistent, it would be much easier. Any ideas?
 
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#3 ·
Sorry about that.

I rebuilt the engine (except for the heads....they were already done 5000 miles before and seemed fine), used all the old igntion parts except I installed new sparkplugs. Things were going fine and I was in the process of tuning when the heat and humidity of last summer hit. I got crossfire and a number of very awful sounding bangs and eventually was only running on 7 cylinders. Found a plug had died. Put a good used one in, still got a bang once in while, so replaced plug wires and then everything seemed fine. I thought I could detect a very slight misfire once in a while after that, but so slight it was next to nothing to worry about, except it wasn't normal.

This year, the engine has been working great, when out of the blue I had a more significant misfire........once. That was last week. Used the car about 3 or 4 times since then and it's been fine.

The problem I am facing is that it's such a random thing....hardly ever happens, plus so much of the igntion systemn has been replaced, that I don't know where to start.....other than checking the plugs, and I don't have much hope they'll show me anything.

I am finding it hard to diagnose as it hardly ever happens...cetainly not consistent.
 
#5 ·
A bad arc....yes, I do believe something is arcing to the wrong place. I am using new 8mm low resistance Accel wires, however it is humid again. I should do the "midnight test" and see if I can see any arcing or corona in the dark.

I have two dist caps (Mallory's), one new and one almost new. Changing them didn't seem to help, but I was thinking maybe the dist cap needs venting, although it never did in the previous 10 years with this engine.

Distributor was originally a dual-point unit, but I changed it years ago by installing a Pertronix unit. I did replace the Pertronix unit last year as well (bad troubleshooting on my part).

This recent misfire happened under fair acceleration.....not quite hard acceleration, but the other little misfires I've had, were after I let off a little bit on the throttle. Maybe this significant one happened when I backed the throttle off a bit as well, but I think I was still accelerating when it happened. Hard to remember for sure at this point.

I know I've accelerated much harder with no problems, like today for instance.

Did it lose power? Well, hard to say as I backed off the throttle completely when I heard it misfire.

I havn't checked the compression yet this year, but it was measuring 205 psi at the end of last year....all cylinders within about 3 psi of each other. So I kind of ruled out a dropped valve seat and things like that. It certainly seems like an ignition problem.

I'm having trouble thinking of other clues as I seem to have tunnel vision on this.....i.e. it must be ignition, but which component?

I'll probably start throwing money at it....change all the plugs for instance. Maybe try those Autolite R45s that seem to be popular with FEs
 
#6 ·
if it was something mechanical it would happen all the time. But it is very damp out there on the rock, so im thinking its electrical too. it would be nice if it started to happen more often so you could track it down. it is a good idea to do the night watch under the hood, hopefully you see some lighting.
 
#8 · (Edited)
Yes, 2x4 Offy single plane divided plenum manifold with twin 600cfm Holley's. I did have trouble with one last year. Loose internal screws on the secondary metering plate (not me that did that). I bought Holley gasket kits (with PVs) and re-did both carbs. Normally they work fine. I forget what the symptoms were that led me to find that internal leak.

Oh.....I used the 6.5 PVs. I know the ones in the carbs were different (lower I think), but I'd have to go look in my parts bin to find the number. Think I am running about 14 inHg or a little higher, at idle. That may explain a bit about the misfire when I let off the throttle a small amount, however that's beyond my experience to know for sure.

Sidenote: I've seen the dyno comparisons, and a Blue Thunder 2x4 dual-plane should work much better than the Offy on my engine.
 
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