Technical HEI, one cylinder has no spark

Hello, I have a 1957 Buick Special that I've been driving/working on since 2007. One of the previous owners installed a low mileage 1985 Chevy 305ci and 700r4, not my choice, but it runs good (when it runs). I rebuilt the motor a few years back, bigger cam, ported the heads, intake, carb, headers, etc. Everything went great, ran good until about a week ago when it wouldn't start for me. I had just installed an electric wiper motor conversion, and in the process knocked the power wire loose to the HEI (didn't notice at first). I think when I went to start it the power wire arced off of the tach terminal frying some electrical towards the base of the distributor. I tested the power source, coil, and then suspected the module (a pertronix unit) so I replaced it with a new extra I had laying around, no luck. At this point I figured it was deeper in the unit, and this distributor was a no name unit that I pulled from a 454 I purchased last year and know nothing about the history.
Sorry for the rambling, just want to give as much info as possible. On to the current problem. I purchased a brand new D.U.I. distributor setup up (cap/coil, everything) from Summit and installed with no issues. New plugs, gapped at .050 as per D.U.I.'s instructions, the plug wires are relatively new so I reused them.
The engine fired right up but ran a little rough. I set the timing, but still runs rough. Long story short I used the clamp on my timing light to test each plug wire and the wire to cylinder #7 either has no spark or intermittent spark. I pulled the wire, checked for continuity and the wire seemed good but replaced it with a new wire anyway. No dice, still the same problem. I pulled the cap to see if there was something wrong with the inside, maybe a bent terminal or contaminants, nothing. Placed the cap back on and still have the same problem. Every other cylinder/wire is firing perfectly.

Anyone have a similar issue before? Or have any ideas? All I can think is to clean the cap really well, but it seems clean already, as it should be. There's really not much to them which is really frustrating the hell out of me. Any help would be greatly appreciated.

 

do you have the wires in the correct order? It's real easy to swap 5 and 7.

might also try a new spark plug in that hole.

just a few lame suggestions, because it sounds like you covered everything else.

 

I see two possibilities, either a problem in cap/wire not letting the "spark out" to the plug, or the electronic pickup, optical, hall or otherwise, not triggering the coil at that one cylinder. Could be a bad trigger wheel (or whatever they're called in distributors) or dirt in the opening it's supposed to sense, for example.

 

I would try a new cap first, could be arcing inside of the cap.

 

Possibly a bad plug wire? Try swapping it with another one and see if the problem moved with it.

 

Well, you could rotate the distributor one cylinder amount, switch all the wires over one and see which way the problem goes.

 

Clock the distributor 45 deg, and move the plug leads around one position on the cap.
Then time it and try it.
If the same cylinder is dead it is either the lead or the plug.
If the problem follows to the next cylinder[same position on the cap] it is either the cap or in the distributor.

 

Here's as good a guess as any, check the routing of your plug wires, you could be getting a miss fire from crossing plug wires. But switching a plug from one hole to another should rule out if it's a bad plug.

 

Why didn't I think of that?

 

Do you have spark at the cap tower for the cyl in question?
I've seen the wrong (long) coil hold down bolts used and the spark was internally arching to the screw.

 

had something similiar--carbon buildup in cap across terminals-

 

Is the tach still hooked up? Probably would just cause a non-run issue if it was shorted from the crossed wire incident but maybe not. The tach has to get a signal. What if that is timed such that it comes from #7 and it just shorts that impulse but not the other 7? An electronics guru like Crazy Steve would have to answer this one. I do know that a tach needs to know how many cylinders it's measuring so it's doing more than just showing every pulse of the coil. I'm out of my depth here anyway, so ain't gonna waste any more of our time ruminating. Watching this thread, wanna see what happens.

 

Checking with a timing light would require the plug to provide a good ground, swap the plug with another cylinder and see if the problem moves with the plug, do the same for the wire.
Inspect the pick up coil gap on the trigger for cylinder 7 if the issue stays after the plug and wire swap.

 

i had the same issue a few months ago . ran like poo so i did the same thing you did after checking my timing i went thru and checked all the wires with the clamp from the timing light . number 7 (just like yours) wasnt flashing , changed out the wire still no flash . turns out it was a bad plug . im running the accel shorty plugs to clear my headers . swapped number 7 with an ac delco r44ts and bango , problem solved .