Intake backfire = vibration?

[MagnaPilot]

I'm not going to lie, there are a lot of things about an HEI igntion that I do not know yet. Long story short:

Carbed 305 with HEI ignition

I had recently changed (most) of my plugs. On the way home from work, one of my plug wires apparently started arching to the headers. This became obvious to me when I came to a stop and the engine did an intake backfire and cut off. It restarted fine, but had an obvious lack of power and seemed to be misfiring. I get it home, and find the bad wire, replace and check the plug. Thinking all is well.

Next morning, take it to work, about 10 minutes into it, the engine developed a vibration that is more apparent at low speed. Even more so if you try to jump on the gas. It sounds as if a cylinder is not firing most of the time. As you sit at idle, you can see and hear it fire off and on, and watch the RPMs jump.

Today, I've gone through everything I can think of. Checked every plug, Ohmed out ever wire, and reseated anything I could, even pulled the valve covers to see if something may have broke there. No joy, the vibration is still there, and it lacks power badly. If you get on it in gear from a stop, it's nearly violent. I'm afraid that something internal may be damaged. What do you guys think, is this motor toast or am I over looking something simple? I appreciate any advice you all can give.

[harleyboy]

well I would like to tell you some thing because mine does the same at low speed and when I hit the gas it takes a second for it to run like a bat out of hell, but I haven't found my problem yet either man.

[Terrybneal]

It may sound silly, but a long time ago, my dad had a truck with a small block that did the same thing. We replaced alot of stuff to no avail. Someone told us to replace the distributor cap. We did, and the problem went away. I have a 4.3 vortec in a s-15 jimmy. The other day, all of the sudden, it started running bad, backfiring, idling rough and wouldnt accelerate over 2000 rpm. Turned out,, guess what,distributor cap. Sometimes, I think we are too smart to look for the simple stuff. Good luck and let us know what ya find.

[coppergmc]

Might have blown the power valve. Make idle like crap if you have a Holley.

[HardinValleyMagic]

I agree with Terry, a tiny crack can be in the cap. Something rather inexpensive to try.

[MagnaPilot]

Update:

Well, I tried all of your suggestions, and thank you for them all. Seems you all were on the right track, but it seems to have been something else.

First I swapped the cap with the one I removed a few weeks back, no dice. I cleaned up the wiring and such, which was a little rusty. No change. Then I went and re-ohmed all of the wires... me being an Electrician, I shouldn't have made this mistake, but, I never ohmed the replacement wire. Low and behold, it was bad, at around 9K ohms, vs 100 ohms of the other wires.... so there was my problem, or so I thought.

I get a new wire (actually rebuilt the old MSD one, worked great). It ohmed out at 100 or so ohms and I threw it on. The problem was still there, but not as bad. I drive it a while, cleaned the plug, etc, still stumbling and misfiring, just not nearly as violent, it was at least drivable now, but still there.

At the suggestion of a friend, I replaced the "control module" in the distributor. Drove it, it was better, just a hint of it being there. So I let it idle for a bit, and then flogged the crap out of it. After the few runs up to 60 or so, it was gone, just like that.... so our theory goes like this:

Plug wire burnt and arched to header. Arching somehow caused a fault in the control module. Replace module, cylinder fires at all times instead of randomly. I could be totally off (I probably am), but at least the truck is running. Just in time for me to speed the weekend doing Drill.... great. I've got 70 miles to drive tomorrow, hopefully I'll make them all. Wish me luck, and thanks for all the help.