Greg Z,
I’m not conviced you have the air flows properly equalized. Your difficulty occurs off idle up 3K rpm. I assume this is under no load conditions?
If so, the engine doesn’t need much throttle opening to maintain 3k rpm. It’s likely you’re on the progression ciruit the entire time.
Is the roughness worse in the off idle-2k rpm range or the 2k-3k range?
If the idle-2k is worse or the same, I think you have an off idle air balance issue.
Balance is very important in the off idle-2k rpm range where you run at small throttle openings, and less important at larger throttle openings.
You state you have a good idle so what may be happening is that as you come off the throttle stop(s), the linkage becomes loaded in a different manner and there is likely enough flexibility in the linkage to change the butterfly openings a little bit and ultimatly the air flow.
When I get serious about adjusting the linkage to overcome off idle balance issues, I balance the setup with the throttle assemblies off the throttle stop(s) and the throttle cable loaded as it would be on the road. Once balance is achieved off idle, return to idle allow the throttle to rest back on the stop(s).
Balance at idle is important, but so is off idle balance, especially up to maybe 2.5k rpm.
Air balance screws on the Weber 151’s, or any other carb with this feature, are normally all closed. They are used for balancing bore airflow within a carb, not carb-to-carb. If there is a mismatch in flow between the 2-bores of a carb, open the lower flowing bore’s air bleed enough to equalize flows and lock it in place. There should never be an occasion where both air bleeds on a carb are open simultaneously. Mark the open air bleed for future reference. Balance carb-to-carb using the linkage adjusment screw.
I not sure it matters much which balance you try to optimize first. It always seems that one adjustment affects another a bit so I go over everything multiple times before I’m happy.
Now you’re ready to dial in the idle mixtures. A color tune works well on the twin cam as one can directly view the flame color. I’ve found it best to be well into the orange range at idle on the pump fuel I use.
I’d be a good idea to plot your advance curve if you ever get the chance but I think you at least need to know the total advance you are getting.
Bill