Engine rough at certain rpm

Nope, its a new bakancer and its a hammery nose rsther than purely a vibration

So if the problem can “come-and-go” by just “pressing-or-lifting” your foot off the gas pedal at a given rpm and road speed range, the two things that change when you do that are:

  1. carburetor fuel circuit
  2. vacuum strength to the distributor vacuum advance unit.

Does the distributor vacuum advance use ported or manifold vacuum? Is it possible you are feeding the distributor with the wrong one?

Its hooked up to ported, so only get vacuum when throttle open

Ported vacuum is at its strongest when you’re cruising along at part throttle (ala 2000-2500rmp @ 40-ish mph).

OK, you say you can get the problem to instantly disappear if you “release” the gas pedal.

Can you get it to instantly disappear if you depress it to full throttle? (since that too should cause your ported vacuum to plummet).

I will try that and report back. Thanks.

Weak spring in the vacuum advance mechanism?

Sounds like a carby issue to me as well. I have the same problem with a MC I own…stumbling at mid-range as it gets off the slow jet and on the main jet.

Of that is the case, bypassing the auto choke for a manual one would easily diagnose this: a bit of choke, applied while stumbling, would smooth it out if it’s a lean mix issue.

I suspect @meanjoe75man is right.
I was asking about the vacuum advance because it’s so easy to diagnose and rule out.

Given it’s a 50 year old carb, the idea that some of the ports may be dirty and/or varnished is worth pursuing.

@1963EJ, have you opened the carb? Do you have any sense if there’s any varnish coating in it, causing guming of some of the passages?

The carb is a Stromberg. I am currently having it rebuilt to see if that cures the problem. I do have a stumble accelerating from zero when in top gear in traffic, and screwing the mixture screw otwards is not havng much effect - cannot get it to “roll” as I screw it out. In fact you can unscrew it all the way and no change to the idle. So I am wondering if maybe its a lean misfire? Ive checked for vacuum leaks using carby cleaner, none were apparent.