So, it's my turn to ask for ideas to locate an intermittent miss.
71 318 mostly stock, very low compression (probably sub 8:1), about 500 miles on re-ring, re-bearing, re-hone on a "rebuilt engine". Summit small style cam, used but checked and indexed at Schneider cams, new lifters, rebuilt 600 CFM holley vac sec, parts store rebuilt electronic distributor. Two interchangeable ignition systems - Mallory HyFire VI AL, and stock junkyard mopar electronic. Mallory Powermaster coil. MSD wires. Coil and modules installed behind the doghouse (A100 van) with firewall passthrough for the coil wire (MS part), other wires going through grommets.

The symptoms:
Intermittent miss at idle, can be slightly felt at higher RPM. Not all the time. Not rhythmic.
What has be done:
New Autolite plugs to replace Champions.
Checked wires leading between ignition modules to distributor pick-up wire. Jury rigged new wires as temp test - no change.
Compression test - 105-115 PSI (low static compression and mild performance cam)
Checked for vacuum leaks.
Checked fo blown power valve.
Put a known well running Holley 390 CFM, no significant change.
Replace distributor cap (both were new).
Replaced distributor.
Pulled wire off one at a time, hard to say, but no change, might try again.
Replaced plug wires with new BWD stock type (to eliminate possible crimping mistake by me).
Vacuum at idle is about 15-16.
In the beginning I wanted to do a leak down test, my buddy said he didn't think it was necessary.
PCV system working well, no blow by, good vacuum at the breather.

So We tried all of the above and it's still there, if you rev it, it smooths out, then after a minute or two, comes back.

My buddy just before he had to leave said he thinks it could be related to a defective lifter or lobe starting to round off in the cam. Oil pressure is good.

Any ideas on what can it be?
My buddy suggested I don't drive it to the Fling (San Diego - LA and back), I think I will drive it.

Thanks,
Ran

I had a valve start going south,did same thing for yrs

71 does not have harden seats so the unleaded is hard on them

by the time I pulled mine,I had 3 bad exhast valves,not bad ...but enuff to leak good

leak down test would be good,only thing you have not did yet

just my

Thanks! Actually the leak down test is the first thing I wanted to do, I will try that.
Any other suggestions?
Thanks,
Ran

Check your distributor for upper bearing slop.
R.

Make sure the coil isn't hooked up backwards.

Thnaks for the ideas, I'll look at the distributors again for slop, but the one we swapped is a new one.
I'll double check the polarity issues, shouldn't be crossed as I made sure they are easily identifyable, One more thing I guess to double check is distributor pickup polarity in the wiring, but again, I tried to make sure that it's fool roof when I wired it.
Would a sticking valve cause similar symptoms if it's just starting to stick?
Thanks for the advice,
Ran

hookup a vacuum gauge,an intermittent drop might signify a sticking valve.Check the air/fuel ratio(if you have a way to do so),Check the ign pattern on an oscilloscope. Let us know what it ends up being.

Now this is just my but in almost twenty years working in a Dodge dealership parts dept, on the tech counter the one sure way to cause a misfire is to use autolite spark plugs in a dodge engine, it doesn't matter 4,6 or 8 cylinder they always caused a misfire, I have tried this myself, my 440 won't run on autolites, it misses like crazy, put in champions and it purrs like a kitten, just something to consider, my .

Haven't worked on the problem, I did drive the van to LA and back. Thanks for the additional advice, I will report in this thread when we find it.
Regarding Champion VS. Autolite - I decided to change the spark plugs from Champions to Autolites after hearing that some Champions cause misses. Anyway, that was not the problem. The problem exists with both brands. I didn't think that would solve it, but I wanted new plugs just to have a clean baseline. In the past I had a bad experience with Accel plugs. Those were total junk.

Thanks
Ran

Don't forget, Mopar small blocks eat their timing chains, some people have claimed full failure in as little as 500 miles...

That's why when I got the rebuild kit from PAW I ordered the heavy duty double roller timing set...also the intermediate shaft is new, but, I know never to say never and look into that, just in case. Thanks for pointing it out.
Ran

Steel gears here, plastic gears weren't considered even for a second.

So, finaly we had time to do a leak down test.
Exhaust valves 4 and 6 were not sealing properly. Since these are 318 heads, I'm not putting any effort in them. My buddy has a set of 360 heads that were done by a so-so shop, we decided against throwing them on there. I will save for a couple of months for a set of Edelbrock heads (which I was planning to get anyway).
Thanks for everyone that helped. And for the record, I'm glad that my opinion that a leak-down test should have been the first thing to do was not wrong.
Ran

did I win?

bad valves was my first guess,

leakdown test comes first always when you cant tune it out with a tune up

cant tune a mech failure out of any engine

Yes you won! Thanks for the input. Even yesterday I was being told "a leak down test wouldn't show us too much", I insisted...and the results were easy to see.
Thanks...unfortunately my wife had to ask how much new heads are.

Ran

glad my 3/4 cam input helped

it would cost more or the same to fix you 30 yr old set...no?

save up!