When driving say average 65mph are you getting any fuel from outer carbs or just center? I'm getting slight surge at 65 or so thought I would step up a jet size or two in center, lower speeds around town seem fine.

hi

might be ignition

try it without vac adv hooked up . might need dist curve adjusted .

what size jets are in center carb ? 63 or 64 normally good .

Also check your initial and total timing without the Vac, advance. S/be around 10-15 initial with a total of about 34-38. WITHOUT THE VACUUM ADVANCE

MSD dist will check timing thanks for the replies

Please do not over jet...63/4's is a not recommended.
62's may even be too large if this is nothing high hp, 61 may likely be required.
The secondaries when closed are constantly providing fuel...always...so they make up the rest of the jetting total - they have adjustment screws to add or subtract fuel
When the secondaries open they provide air/more fuel and of course the OH WOW! acceleration. Let me see that happen with efi or a 4bbl....I'm waiting

More than likely the six pak set up and the over all tune up is not Goldilocks [just right] so there are problems like surging etc etc etc

Which MSD distributor, self contained or with one of their separate ignition boxes, ECU?

stand alone no box not a fan= [fazing]and all

[spoiler][/spoiler]Sounds like it might be the distributor trying to add timing at cruise RPM
I try to recurve all my distributors to get all the mechanical advance in by 1600 to 2000 RPM max
Have you tried tuning the outboard carb idle fuel ratio with the idle fuel adjuster screws in the base plates? If not try gently closing them until fully seated and then back out from 1/3 to 1/2 turn out from fully, seated depending on what altitude you're at
Six packs rule when they are tuned properly tuned : scope

Wrong gas cap or gas tank venting issue?

the vacuum can is adjustable with a 3/32 allen key inserted into the nipple end. have you tried adjusting that ?
you could have too much vacuum at cruise, causing an over advanced ignition curve.

no vac advance promax metering plate ,rear base plate and jettable plates in outboard carbs .where should you get vacuum with promax center metering plate

That’s a can of worms! Factory was metering plate. Some people like manifold vacuum. I don’t have my vacuum advance hooked up either, no problem

If you're not running the vacuum advance, move on to checking the initial and total timing. Might want to verify the timing mark is correct when at TDC