Recently rebuilt my holley 750. Starts and idles perfect.Floats are set so fuel is at the bottom of the sight glass. Idle mixture screws are both one and a half turns out. It seems from 20 to 45 mph it stumbles, kind of sputters like it wants to die. After 45 it runs smooth and throttle response is better. I`m assuming the accelerator pump is not adjusted correctly. I`ve tried adjusting it and can`t dial it in. 440 engine 6.5 power valve,jets 72. watched a few youtube videos and can`t seen to get it right . Can some one point me in the right direction here? As always thanks !!

What's your timing at? Trans kicking down?

I ain't a holley guy but iirc they want 15 tho clearance on that lever. I would just see how much squirt you have idling and at the RPM you would be at, at 25-45 MPH (you would not be under a load but it'd be a start). No vac leaks. I'm kinda WAG but that'd be where I'd start

Yes robert i have the lever set at.015. Timing is slightly advanced. Not sure exact number. Trans kickdown i`ll inspect linkage.

I'm trying to think what is different after 45 MPH than 20-45. & it runs good after 45 or just so so? Vac leak keeps coming to mind. EDIT I reread & you rebuilt it, was it OK before as far as this issue goes other than needing rebuilding? If so I would confirm all vac lines are OK/base gasket correct/check bolts/screws for no loose ones then open the carb back up. if no on the Q, holler back

72's are kind of small for a 440. what are the secondaries? What is the car and build? Squirters?

that .015 clearance is when the throttle is fully open.

when your throttle is closed you want the pump lever adjusted so that there is no slack, you want a pump shot as soon as the throttle makes any movement.

if the stumble comes in when you are keeping the throttle steady then the pump shot isn't in play there.

did the kit you used to rebuild the carb give you the same power valve rating as you had before???

if you don't have a large cam then you can probably go up to atleast an 8.5PV...

I have it adjusted at .015 at idle NOT wide open throttle. I will adjust and report back.

sounds good

you need to get your baseline setup correct, the transfer slot should be square on the primary side. then hold the carb up to light and adjust the secondary blades to mimic the primary by the light peeking through

hopefully from there your idle speed will be close to where want it.
set timing, fine tune idle speed then adjust idle mix (smooth idle highest rpm you can get out of it, or by highest vacuum), fine tune idle speed again.
hopefully you are about 1.5 turns out on the screws.
make sure there is not slop in your accel pump linkage.
if it seems lean under light load then put a higher rated power valve in it until it smooths out.
finally start tuning your primary jets, then move onto your secondary spring rates...

""It seems from 20 to 45 mph it stumbles""
""After 45 it runs smooth and throttle response is better""

If it stumbles while cruising (not accel), and after 45mph the cruise is smooth, it sounds like your transition circuit (IFR's or IAB's) are lean. Then when it starts transitioning into the Main circuit, your Primary Jets/HSAB's are helping as they should. Try some smaller Idle Air Bleeds to see if that helps the 20 to 45mph stumble. This picture shows you which bleeds are the IAB's.

Note:
See above response as to getting the timing/idle setup correctly first before tweaking the cruising tuneup.

The reason for the .015 @ WOT is so the acc pump doesn't top out and bind/bend the linkage.

Two main reasons for stumble
-not enough fuel
-incorrect timing

To avoid an over-rich carb, correct the timing first, then make carb changes.

Is the stumble new? If so, go back through the carb again with special cleaning attention to the idle, transfer, and primary main circuits.

More initial jet and larger squirter. After your timing is dialed in.