750 + 400 ci = too much (now with vid)

I have been plagued with this problem since I built the motor. The motor is a 400 stock bore, 256 comp cam, manifolds pretty much all stock.
Well I have a bad stumble right at 950 t 1100 rpm's. I have adjusted timing up down etc. Changed the pump cams, bigger seems crate a backfire. no difference with a smaller cam.
I reused this carb from 440, doesn't seem like I had any problems when I used them together.
Thought it might be timing, but still cant get it tuned right. Any ideas -
Oh yeah it screams after 1100 rpm's and runs great
Thanks

holley? check for a leaking powervalve. if thats ok check the idle mixture.
the transition from idle circut to the high speed circut can be a pain if not
properly tuned. holley's never stay in tune for long.

what carb? If it's a holley you might need to change the PV...stumble usually = lean but not always. Have you hit it w/ a vacuum gauge?

Thanks, I forgot, yes it's a holley. it sat on the shelf for a few years. Seemed okay. How do you check the power valve.
I used a vacuum gauge to tune it with, I had leaned the idle circuit until I used the gauge, had to fatten it up some for highest reading - Thanks

well I guess easiest way is to suck it...the PV of course. If it opens it's ok if not it could be bad. I'd guess w/ your set-up a 6.5 might work???

""Well I have a bad stumble right at 950 to 1100 rpm's.""

You need to hook up a vacuum gage to read manifold vacuum while you are driving. Just add a long piece of hose from the carb into the cab so you can watch the gage while driving at the 950 to 1100rpms. Then you can rule in/out the power valve.
Off idle rpms like 950-1100rpms would point to needing a larger squirter size to me. I would try the next size up on the primary squirter and then retest the pump cams.

first,ck the pump arm and adjust. many times holleys get put on the bench,or shelf,and the pump arm[s] get bent,and you get a mis-adjusted pump clearance. w/o proper adjustment,it will have a stumble off idle,w/o a "pump shot" to raise rpm's. make sure there is 0 clearance at idle,and NO bind at WOT. check manuals for full descriptions on adjustment. also ck mech advance,and is the timing at 35-38deg all in at 3k rpm or so? any vacuum leaks? lean cond will cause soggy low end also...good luck,mike

Well first that is basically a stock cam. two big things with a stock cam first if you have the vacuum advance on manifold vacuum well just have a nice life.
second with a stock cam likely pulling 14 to 16 vacuum in gear idleing you likey need to go up to at least a 8.5 power valve

Is it a DP vacuum secondarie or one other low budget things they sell? # would be nice

dual feed vacc. secondary 4160
One thing that might matter, is I pulled apart an old 400 dizzy that had lighter springs as the 400 i have had a centrifugal advance ( i think) not sure on the total timing, just have a standard light, plan getting tape to check it, initial timing is around 12-14.
I set it here, drove it, and backed timing down until I had a good combo. of driveability, starting and no pinging...
Thanks - Mike

what's your in gear idle speed? and in gear vacuum? What # power valve. yes as said add a vac gauge & note the vac when it hesitates. Is this from part throttle or stomping on it? ported or manifold vac adv? EDIT any possibility of a vac leak?

A 750 carb is not too much for a 400 inch engine. You might have some carb problems with your specific carb but there isn't a general issue with that size of a carb on that size of an engine.

Thanks for all the input,
I hope I have a vid that might help out
just ignore me being stuck and the loose belt
I'll double check the carb when i get it out

power ram

I went through the same symptoms you are describing and it turn out to be dirt in the carb. Also like stated check your vacuum in gear and half that should be the power valve size. Take the air mixture screws out and run some carb cleaner through it.

Also,
Don't forget to check for a vacuum leak at the throttle bushing.
Jim

Alot of things can cause a stumble and most of the time it's a lean condition. But also make sure the idle is not to high that the transfer slots are uncovered in the carb. This can cause a problem as it will have you leaning out the idle screws since the transfer slots are already starting to pull fuel. Then when you step on the gas you dont get the extra fuel that is used in transition from idle to main metering system. Thats why alot of guys drill holes in the throttle plates to give it the extra air for the higher idle but still keep the throttle plates closed enough. We use a 750 DP on my sons .030 over 400 in his Dart. He tried and 830 anular carb and an 850 DP but it still ran its best with the 750 DP. Ron