Holley stumble trouble
Seems like the transition is not lasting long enough until the mains can take over.
If the throttle is low enough, then this could be basically true. Lasting long enough would only be if increasing the throttle. But lets say you start to give it a hair more throttle and hold it. Is the delay the same or does it continue?
Subtle difference. If its a delay, then it might be related to the initial transition slot relationship.
If it continues, it seems to me more likely its the idle system choking out of fuel at the top of the transition. Also could be a goofy block with a kill bleed in the dogleg or something like that.
Anyway, what I'm thinking is that if restricting the IAB didn't make it richer, then at that throttle position and vacuum, the IFR is restricting any additional flow.
Whether the correction should be done there, or it would be worth trying to ge the mains to start earlier is hard to say.
This is where a WOT 3rd gear run to max Hp with a WBO2 logger can be useful. it would indicate whether an increase in MAB as YR suggested might be needed for the top end. And if it did, then I would try that first. If not, then I might try the larger primary IFR first, depending on what we find in the block.
