68 Monaco, All stock pretty much untouched except for an edelbrock AVS 650. Timing was set at 14 initial, ~20 mechanical, and 18 or so vacuum ported. petronix conversion.
Car backfires through the carb horribly when at low speeds and the secondaries are opened. Adjusted the air valve as tight as possible with little to no help. tried manifold vac. for the VA and disconnecting it completely.
Timing chain was loose enough to be making noise barely touching the cover. It was swapped with an Edelbrock set.
Very little change.
Swapped to a performer 600, better but still bogs / hesitates when the secondaries are opened.
To shorten the story a bit, Also tried a 1850 Holley, better but still hesitates a bit. Increase timing to 20, better, 30 even better, settle on 36 INTIAL 😳. TDC was verified when timing chain was installed as well as damper to timing mark.
Runs awesome at 36 +20M +18 on ported. Timing light was also verified.
Any clues on this one??

Sounds a lot like my 440 I was having problems with. Finally figured out timing tab is in the wrong spot. With the TDC balancer mark at 6 o'clock I taped the balancer and measured to the 0 degree mark on the tab and turns out it was 23 degrees off from where it should be based on comparing it to other big block timing covers I have. Had to time it with a dial back light but it runs great. Shows 38 degrees initial which in all actuality is only 15 degrees.

hi

cam retarded , timing chain off 1 tooth !

Has got to be one of the two responses so far.

Compression test? I'm thinking burnt or sticky valve.

Is the polarity correct on the magnetic pickup? I've seen it retard the timing by as much as 20 degrees on an MSD ignition.

Absolutely have had that problem. Around 2000 or so, many of the MSD's coming out of Mexico were put together by people who were color blind. Got that information direct from MSD rep at Motor State Distributing. Solution was to cut one pair of wires and re-attach them flipped. Previous to doing that, the electronics had the distributor retarding when it should have been advancing. BTDT.

This, if you need 36 degrees initial timing, you're off a tooth. If that's not it you've bent an intake valve when the other chain was failing.

I have to agree with with everyone on the timing issue, be it ignition of cam. The car belongs to a friend and came to him this way. Running, but would not take a quick WOT without dying, or backfiring through the carb. Through the course of diagnosing the above an erratic squeaking started that he attributed to the belts.

It started to become more noticeable/irritating so he removed the belts and the noise was still there. tracing it to the timing cover he installed a new Edelbrock set.
If he hadn't just put a new chain in it I would say pull the cover again to verify that and suggested doing so. Knowing him well, I am 99.9% sure he installed the chain correctly, but it's possible he missed by a tooth. He has decided to run it as is.
The ignition is a petronix unit and wired correctly.

Myself, I would have to pull the cover to verify and /or keep digging to find the cause

I do want to thank all that responded

If it's a multi keyway timing set, are the right marks lined up? BTDT

it is and don't know but I'm going to say the answer is likely the valve timing. he's choosing to run it as is

It won't fix itself, and has to be taking all the enjoyment out of driving it, which is a bummer. I'd be worried about it quitting in traffic ect., and being in someone's way since it's not going to accelerate smoothly like normal. Who wants to drive around with that on their mind?

Agreed he says it runs fine and he's happy so

talked with him again today and relayed everyone's comments. He said he's going to pull the cover again when he gets time as it is bothering him as well. I'll update if and when he does
Thanks again to all

Let him know the crank key doesn't point straight up. That's what got me.

I had a set one time that had a multi key way that included a circle. It looked like a dot to me. Degreed it and was way off. Then saw the little dot. Used that one and it was dead nuts on.

he said the the crank keyway was ~ 2:00 which I believe is correct ?

I hate the 3 slot crank gears and their markings 🤬 Easy to get balled up if not paying real close attention