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Discussion Starter · #1 ·
Greetings forum, a little back ground, then my question.

Got the car: a 1970 L78 SS from my stepdad, he had not driven the car for 10 years or so, just started every year. it has a late 70-71 396 (not original). I have changed the plugs (twice) and wires (MSD), put an HEI, changed the radiator (original Harrison was kept and will be re-cored) and all the rubber hoses under the hood, put on a new fuel pressure regulator (old one leaked), and filter, had the carb rebuilt (Holley 650 db), new air filter and oil change. I had the valves (hydraulic lifter) adjusted and have got it running pretty well. The other side (even) do not have a huge difference in temps.

I got a little laser temp gauge (harbor freight) and was messing around, I noticed #5 cylinder is 100 degrees or so cooler than #3 and #7. I have read that while idling (which it is in the pictures) that is no big deal. but I would like to check with you all. pics are of #3, #5, and #7 with temps taken in the same spot on each header.

my question is, is this something I should worry about?

I look forward to the insight.

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I had the same thing happen. Diff was a bit more than 100*. Compression test revealed that one was down 50%. Turned out to be a broken ring land.
 

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You've got misfiring on the "cold" cylinder. It could be a bad spark plug, a bad plug wire, or a bad dizzy cap.

I've been meaning to write this up, but I had a similar issue with the straight 6 (250CI) engine I've been taking care of and doing "performance upgrades" on.

I had multiple cylinders that were cooler, and I had intermittent misfires (on those cooler cylinders). It turned out to be the dizzy cap was defective. Actually, I tried 3 different caps, and all of them had misfires on multiple cylinders.

The first thing I did was hook the inductive pickup clamp of my timing light up to each plug wire, pull the trigger on the timing light and just shine it on the valve cover. The "hot" cylinders had a steady, regular flashing pattern. The cooler cylinders would "skip" from time to time, indicating occasional no-spark misfires. On one of the defective caps I had, the #6 cylinder was extremely cool, compared to the hottest ones, and that one, the light rarely flashed, with occasional bursts ranging from a single flash to maybe 5 or 6 flashes in a row.

Here's a pic of one of the dizzy caps:
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On that cap in the picture, only #3 and #4 cylinders (the two on the sides) were firing consistently. Those were the hottest cylinders. The other 4 were having occasional misfires. Notice the weird angled bends in the inner part of the terminals where the rotor makes contact. Only #3 and #4 are close to straight (or, more correctly an arc that matches the path of the rotor tip).

In the end, I tried 3 different caps and 2 different HEI dizzies for the coil-in-cap design, and 3 different HEI ignition coils. I used all 3 caps on both dizzies, and I never found a combination that didn't misfire on multiple cylinders.

I finally punted with a entirely different new dizzy and cap with a center terminal and the coil mounted more conventionally, and a MSD "Blaster SS" coil. For the straight 6, there's a HEI dizzy available for the 1975 and 1976 models that did not have the coil in the cap, and that's what I used. Replacing the dizzy, cap and coil with a different design fixed everything for me.

For a V8, you may have to go with an aftermarket/performance magnetic pickup dizzy with that coil. You may even need the full "red box" MSD D6A ignition controller.
 

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Discussion Starter · #7 ·
Nice write up, I have already installed a HEI distributor, new MSD wires and plugs (twice). This is great info thought. Thank you.
 
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