engine stumbles right off idle

[culp71]

freshly rebuilt everything, ran perfect for 450 miles,
now the engine wants to stumble and hesitate right off idle, when in gear.
essentially stock 71 340 w 600 Edelbrock.
adjusted idle mix, without change. otherwise runs perfectly.

thoughts?

[rebelyell]

Have you re-torqued everything?

[Rich G.]

Probably the accelerator pump. That's what give you the shot of fuel when you step on the pedal. Some are adjustable. Also ck the ignition timing.

[Chryco Psycho]

I know where the problem is exactly , 1406 Edelbrock
Check your float level I bet you did not do this before installing , , they call for 7/16 I like 3/8 better
these are the worst excuse for a carb out there ...
My friend tested a 750 AFB Eddy one on a dyno vs a 750 Holley HP series , @ Wide open throttle the power was down 5 Peak Hp , no big deal right But at 2500-3500 where you do most of your driving it was down over 50 RWHP 

[tman]

Try the ez stuff first.  Accelerator pump adjust, try moving the pump link another hole.  See video at 7:35.  This video will also show how to check floats too. 

https://www.google.com/search?q=adjusting+edelbrock+carburetor&rlz=1C1CHBD_enUS752US752&oq=adjusting+edelbr&aqs=chrome.0.0j69i57j0l4.8251j0j8&sourceid=chrome&ie=UTF-8#kpvalbx=1

[nsmall]

I have a Carter AFB, hope to change one day.  I think it is a 600.  I have a mild built 340.

I had a similiar problem.  Changed the squirter AKA nozzle from a 28 to a 44 (the biggest I could buy) and it helped a lot.  Actually proper timing thanks to @Chryco Psycho probably helped the most.

Everyone has opinions but Chryco Psycho words are gold in my book as every idea I have used of his have helped my car run way better.  I recommend reaching out to him.   

[Topcat]

Also get rid of the leather boot plunger if it has one.

Newer one's are made for alcohol in fuels.

Leather one's dry out and go bad over time.

https://www.ebay.com/itm/Edelbrock-1470-Carter-Carburetor-Carb-Accelerator-Pump-Plunger-Hi-PerformanceA42/192600374038?epid=1211761390&hash=item2cd7e07b16:g:qikAAOSwPhdU8TKL

[culp71]

alright I will start with replacing the accelerator pump, as since it was running perfect all along and suddenly changed...this makes the most sense.

do accelerator pumps normally fail this fast?

[73_Cuda_4_Me]

Also check the step-up pistons... I had one of mine stick down in the closed position. I found some fine silt in bottom of carb, even though I run 2 filters. It also plugged one of my idle circuits, so that changing the idle mix screw wouldn't change anything on driver's side...

[culp71]

removed carb top, replaced stock 31 accelerator nozzle with a 43, and changed out accel. pump, the stock did not seem to fit very well and no "vacuum" could be felt when I would plunge it, the replacement fit tighter and seem to take care of the stumble issue. I tried altering timing, but either direction made it worse. everything else seemed perfect. hopefully this isn't something I have to change out every 500 miles, as removing entire shaker assembly sucks.

[Rich G.]

The crappy fuel they have now isn't going to help the situation any either unfortunately.

[Rumple]

I have a 1406 on my 340 (for now) and I've been using info from here to calibrate it.  So far I only have the springs, rods, and jets from the 1405 swapped in.  I did take it for a short drive and everything seemed OK. Need to find time to play around with it a bit more.