351w hesitation at part throttle

[Nuzzlet 0]

I have a 351w with a roller cam in a 1969 Mustang. Vacuum secondaries. 

At half throttle everything is great & there is no hesitation. But when I go over about half way there is a stumble right before power kicks in. My best guess is it has something to do with the vacuum secondaries. I'm not really sure though. It doesn't seem to be accelerator pump related. Plus, when I rev the engine in neutral there is no delay or hesitation whatsoever. 

Any ideas? Thank you!

[Corbins 2]

Looks like a Holley 650 ??  Yes , you can change springs in the vacuum secondary, Holley sells a kit as does AED I believe. It ,ight also be a power valve issue, you'd have to check how much vacuum you have and compare that to the appropriate power valve required.

[stuart 13]

I'd check the clearance on your accelerator pump arm and also see what size power valve you're running.  You may need a power valve with a higher vacuum opening to give the carb more fuel when you're transitioning from part to full throttle.

Stu

[Vicfreg 771]

You also might want to check the secondary throttle plates to see how much of a gap there is.  A lot of times carbs can be set up to idle correctly without properly adjusting the idle.  This can sometimes result in compensating for poor idle by cracking the secondary throttle plates.  This can cause problems in the transition when the rear throttle plates open.

There is lot of literature/help on the Holley website.  I always baseline my carb by setting the idle mixture screws with a vacuum gauge, then using the curb idle adjustment to get my final idle value, instead of fooling with transfer slot/gap  adjustments on the rear throttle plates.