I recently swapped cams and ran into problems with my Holley 750DP carb see post https://www.thirdgen.org/techbb2/sho...hreadid=249276

I just bought a new 750DP carb from Promax, its a 4150 style carb similar to the Holley Ultra series. Looks like this http://promaxcarbs.bizland.com/carbs.chtml This carb has a lot more adjustments that can be made idle air bleeds, high speed bleeds, idle feed restrictor, emulsion jets. Stats for the new carb: 74/84 Jets, 4.5 power valve, primary& secondary shooter 31, pink pump cam.

My timing is locked down at 36 degrees, I have 10in vacuum in drive at 850rpm and 14in vacuum in park at 1050rpm. Mixture screws are set 1 1/2 turns out. Backing out the screws until rpm picks up is at about 1 turn out but car will hesitate at this setting so I backed out the screws to 1 1/2. The car runs better with this carb but still has a part throttle stumble or flat spot. At about 1/2 throttle it does not want to accelerate but is fine at less throttle or full throttle. I've put in 77 idle air bleeds in to help the richness at idle and switched to a hotter plug Champion RC14YC from NGK R5671a-7. Idle is still rich but not as bad after these changes. Power valve is a 4.5...is this too low causing part throttle problem? or could this be something else? I've experimented with the mixture screws, timing, and throttle blade positions and can't seem to get the part throttle stumble out. Thanks in advance for any suggestions..and sorry for the long post.

 

Update, I changed the power valve from a 4.5 to a 6.5 and most of the part throttle stumble is gone. I think the 4.5 power valve was causing a lean condition at part throttle. I may try a 7 or 7.5 and see what that does.