Cylinder #3 no spark
 

Car is my buddies 70 Lemans. Its got a 455 with a roller cam, out of the box e heads, perf RPM with 1" spacer, and an 850 holley DP. Oh and dougs header. At Atco saturday in nice air car runs a 12.10 at 114, its been 11.69 at 116. Since Norwalk the car slowed down after the 2nd run there. We rejetted tried different timing and so forth and finally on Saturday we found it, for whatever reason we pulled #3 plug. Cylinder 3 has not been firing for a while. This is a full MSD with digital 6AL and a pro billet dizzy. So we take #3 plug and hook it to #1 wire and have spark, OK... Check cap for cracks and OK there. Check rotor tang height all good. Ring out wire and its fine. So we take the car home and get after it, the box, timing control are 100% working fine. So we remove the dizzy and clean up the 8 tabs and the magnet and measure the air gap on all 8 and they are the same. The magnetic pick up reads 580 ohms. Could the pickup be bad even if its reading 580 ohms? and why #3? There was a little corrosion but no where near that I've seen on working ones. Anyone have any ideas? There is fuel getting to number 3.

Bad plug wire on #3? Have you done a compression check?

Never seen a pick-up coil cause a single cylinder issue. Typical spec. is 400-700 Ohms. So you can eliminate that item.

Logically, it HAS to be a part that is only used by #3 cylinder.

Spark plug - you already verified that it works
Plug wire - ohm check and visual inspection isn't a 100% guarantee
Distributor cap - I've had caps before that looked perfect but caused squirrely problems.
8-tab wheel in distributor - visual inspection should be pretty reliable if all tabs look the same and gap measures the same.

That narrows the fix down to a new wire and/or a new distributor cap.


Eric

Well, heres what happened... I verified every wire and the cap last night. My buddy put in the pickup. MSD says between .018 and .030 air gap, we installed the dizzy and had no spark anywhere now. Coil was firing. Remove dizzy again and adjusted the air gap to .020 and she ran. Firing on all 8 cylinders reset timing to 38 and road tested. Runs way smoother now, weird you really couldn't tell one was not firing until you get all 8 firing.

I initially put the air gap at .028. Thats when I had no spark everywhere. Thank you for the the replies, I cannot explain it.

Glad you solved it. Must be something about #3 that takes more voltage to fire than the others.

Eric

Could be Eric. Just a few observations about these distributors over the years. I switched to all MSD stuff in probably 2005. I put on a 6AL, timing control, and the 8563. About 3 years later I removed the engine for something (cant remember)but I took out the dizzy and this thing was so corroded I thought about buying a new one but I cleaned it out and drilled a 1/2" hole in the cap trying to vent it. Probably 5 years later I took it out again and wasn't as bad but still LOOKED like it wouldn't run. Thought about buying a new one again but cleaned it up, new cap, and rotor and ran it. Same deal in 2020 when I put the roller cam in, I never replaced the pickup on mine in 17 years! In the interim years there I've put 4 MSD systems on friends cars. Two fox body stangs, a BB 70 chevelle, and a friends nova. I am not a pro mechanic just a guy that likes to read and wiring scares some people. Every one is still in the car and running but my one buddy sold the Nova. So here we are with one that is 1.5 years old, barely any corrosion and it presents us with a not so good track day. Especially when we lightened up the car, we wanted to see a difference. So Wednesday I read the entire MSD tech guide and it says to drill 1/4" holes on the plane of the spinning rotor so we did that. Will it make a difference?

On a V8 engine with two rods on each rod journal it can be not so obvious at low rpm if a cylinder is not firing when you have a motor rocking around from a big cam anyway.

Also magnets don’t last forever, so as there level of Gauss dwindles gap settings will need to change.

take a ohm meter and check resistance between plug body's and head #3 might have higher resistance due to paint or something like that creating a incomplete circuit