I have not given up - like the mid range performance improvements even though the stumble on takeoff is driving me crazy.
Changed to Accel. Pump cam from white to orange. The orange cam has the 2nd longest shooter time of the 8 available and the 3rd largest cam rise. Also adjusted the throttle lever to have zero clearance with the pump arm. Adjusted the idle mixture screws to give max. manifold vacuum reading at idle (18 in. Hg.).
Less stumble noticeable but still there. I now plan to increase the shooter from 25 to 28 or so to see if that helps.
The power valve was changed from 8.5 to 9.5 in Hg. opening but that did not seem to do much.
Thanks again for all the inputs. I'm learning something useful.
I had a stumble with my Holley 390 as well. After going through the carb and getting it fully rebuilt professionally by a racing specialist of some note, I moved on to related systems that can effect drivability. I discovered that my ignition timing was good and under normal operation, it functioned properly. It wasn't until I had a chance to see the vacuum advance in action that I discovered that on moderate to hard acceleration the timing was dropping due to a loss of vacuum. It was only for an instant, but it caused the engine to bog badly for that instant.
I added a small reservoir to the advance system and the problem went away. Don't rule out a timing issue!!
