[JohnD1956]

Update, This car is gonna get killed. That's right, I'm gonna beat it to death with a ball peen hammer.

Today when I started it, it had a awful miss, and it is still missing. I went over all the vacuum hoses, all the distributor things, all the wires, and added dry gas just in case that was the problem. I even sprayed all the distributor internal parts with WD 40 and nothing has stopped that miss.

It does not miss at idle, but even a little bit of a load has it "poop poopingh ".

It feels like the pick up coil miss but that's just two weeks old. I took the spark control module to NAPA where they ran a diagnostic test and it passed with flying green colors. But it's still missing.

I haven't picked up the spark plugs yet and now it's supposed to rain the next few days anyway. Sheesh, this is frustrating!

[simplyconnected]

Check distributor for loose grounds and re-seat your connections. No missing at idle, and intermittant missing at ~45 usually means a weak electrical connection somewhere around your distributor, coil, etc. Remember, two wires make the path. Many times it's simply a ground connecton or weak pin in a connector. Check pins for witness marks of heat (melting). Coils tend to show signs of failure at high rpm before they give out.

[NTX5467]

I can't think what would be really different in below 45mph and 46-55mph, in the way of carb operation, engine load, etc. The increase in engine rpm would be very modest, so that particular speed break seems unusual.

If there were any index points on the ignition part you replaced, did you get the new one indexed at the same place? What might be happening is that if the index point was not correct, it might be acting like a set of points that's set with insufficient point dwell . . . which would mean "weak spark" as the vacuum advance moves the breaker plate and/or the advance weights do similar with small increases in engine rpm. Just a stray thought . . . but what about the ignition module (which controls what we used to call "dwell")?

Can you "drive through" the random miss with more throttle input? Just another stray thought, but anything that might have a vacuum leak or is controlled by vacuum and is not working correctly should be causing issues well below that rpm/speed range. Can the condition be found if you lock the trans in 2nd gear and hit the same rpm levels?

What about the plug wires? When were they changed last? Putting new plugs, or cleaning and gapping the existing ones, will lower the energy required for the spark to jump the gap, which might mask a wire issue until the plugs get some wear/miles on them. If that's one of the engines that had a spec plug gap of .060"+, you might try them at .045", which will also lower the voltage requirement to jump the gap a little.

What about the inside of the cap and the rotor? Deposits on the rotor end and plug wire terminals inside the cap? Just tryning to think of things, which are somewhat normal "wear" issues, which might cause issues when a "build tolerance stack" situation might happen.

NTX5467

[JohnD1956]

Thanks guys, for those good ideas. Last night I checked for arcing wires after dark. While at idle I didn't observe any leaks. I raised the idle while in park to almost 1700 or so RPM and no arcing. Then I had my son put it in drive and raise the idle just enough that it started to miss. At that point I could see the arcing on cylinders one and three just around the edge of the plug boot.

I'm back to the plugs being the root cause of these problems however, if this does not fully fix it I'm going for a new coil next. Then it's off to a competent mechanic for further testing. I got new plugs today but the rain stopped me from putting them in.

Couple of things I just wanted to mention.

I rechecked all the connections yesterday when I reinstalled the spark control module after having it tested. There is no evidence any of the connections have failed or overheated.

And the pickup coil can only be installed one way. There is a pin hole for the vacuum advance hook up and if you don't put the coil in the right way this won't align. I think this negates the theory about the dwell being misinterpreted.

However I do think the theory about the weak spark is correct, and the possibility of a failing coil is right up there. It ran pretty good when I got it six months ago but it is 31 years old.

Eventually we'll get this figured out. I put the ball peen hammer back in it's place.

[simplyconnected]

Originally Posted by JohnD1956

...Then I had my son put it in drive and raise the idle just enough that it started to miss. At that point I could see the arcing on cylinders one and three just around the edge of the plug boot...

Good job, John! By George, I think you got it. A shunted arc is certainly a low voltage condition at the plug gap.

[Smartin]

Make sure your distributor shaft doesn't have any side-to-side play. I chased an intermittent miss for 2 months before I found that problem. I could "drive through" it, but it progressively got worse, as the distributor shaft bushing ovalled out.

[JohnD1956]

Hey Adam, thanks for that tip. How odd that problem. How did you figure that one out?

Meanwhile, let me be clear that I am not saying the car is definately fixed. I learned that particular lesson. However I replaced the plugs today and the miss is gone. I hope it stays that way but I want to put a few hundred miles on it before I'll feel comfortable with sayings it's fixed.

Meanwhile at the risk of embarassing myself I should mention that I made a mistake when I cleaned and regapped the old plugs. The #2 plug which is under the compressor was not tight. It appears I hit the bracket for the compressor with the ratchet and I must have thought the plug was fully seated. I'm lucky the thing did not blow out of the head.

Anyway I made sure all the plugs are now fully tight, and put fresh grease on the boots and the engine feels good. Now I' just gotta see if it lasts.