63mako

Do you have the 670 Holley street avenger? Pretty good, versatile carb. Probably needs tuning to your engine. A dyno tune with a good wide band and good carb tuner will get you more than a out of the box carb you have to tune to your car anyway for about the same or less money. If you are handy and want to do this yourself and learn this is the link. Follow it through before you go drop $500 on a new carb. Most muscle cars had a dual plane and vacuum secondary carb out of the factory. Even the High HP, High compression cars. The 1970 LS6 454 ran a 780 VS carb, low rise dual plane and red lined way above yours with a pretty nasty cam.
http://www.nastyz28.com/~ericf/tech/htune.pdf

woodlandcorvette

I run a 770 sa on my 383 440hp 480 ftlb and I had a off idle stumble also. What cured mine was to change the power valve from a 6.5 to a 4.5. I had low vac at idle and the pv was kicking in to soon lower the powervalve cheap fix.


have fun with new motor

Shark Racer

Venturi effect basically means that you'll get slower intake charges out of a carb pulls less CFM than your engine needs. This is really important because it slows down how much fuel actually makes it into the motor. Even if you tune a 750 perfectly, you're getting less intake charge overall as the motor simply can't pull as much mixture from the carb as it could with a proper size carb.

This is negated with FI setups, which is why we have giant throttle bodies on lower CFM cars.

Go with the 650. You won't regret it.

DaJesterMan

Trust the Holley Tech, they know their products... most people over carb their engines and think its going to be better, then they recommend a larger carb to other people and tell them it will be better. Like everyone said, 650 will help with low end performance due to the higher velocity of the air coming in (venturi effect) which will atomize the gas better. Only at very high RPM's will the 650 fall off because of restriction, but 90% of people here don't run at high enough RPM's for long enough for this to be a factor. (I know I don't) The 750 will also require a more significant jetting adjustment to tune... so the 650 will prob run better out of the box.

BLOCKMAN

I agree with a 750 your not going to see any gain because your not going to be moving 750 CFM of air with a 420 horse 383.

We have done some mild 383 builds 450 horse and 500 FT pounds of torque and on onewe used an HP 600 and tried a 750 results were about the same.

The 650 may give you more throttle response or the 750 carb.

You really have to have some big heads and cam to make a 750 carb work to its full potential on 383 CID

forvicjr

Well find you a good carb builder and have a custom carb built with 650 main body and 750 base plate. have the air bleeds tuned to your combo along with all the other advanced adjustments thats needed. No off the shelf carb is going to boltup an run perfectly. The 650 body /750 base will up the air flow and give you a happy medium. I have done this, it makes an awsome carb. I never buy a new carb i build all mine from the holley carb and parts collection i have...

73StreetRace

Both carbs will work if they are correctly tuned, but the 650 will probably be very close to perfection right out of the box.
It can be a little more complicated but still feasible with the 750.
If I had to make the choice I would try the 650 except if the car is mostly for a drag/strip use.

tt 383

What manifold do you have? I have seen a 650 only be down 7-8 hp on a 500 hp 355 @ 6400 with a Performer RPM. What was lost below 5000rpm (20 hp & 35 ft/lb @4200) was very noticeable though. Your engine will run differently in the car as opposed to what a dyno showed, dynos have unrestricted inlet air and exhaust, usually not run with accessories or water pump or the same advance you will need in your dizzy. Their is reasoning that a dual plane can be overcarbed because you only feed from half the carb, but where the engine runs mostly at you will feel the difference between a 750 and a 650 if they are both tuned correctly.

DRIVESHAFT

The 650 and 750 have the same base plate. That would be a bit of a waste to swap.

The 670 that you already own can be tuned to work properly for you. A double pumper is never required just to cure a stumble.

If you do decide to buy a new carb, I would buy the 750 if you have a dual plane intake.
If your 383 is lazy at low RPM with a 750, either your tune is off or you have a bad engine combo.
Remember, Chevy put 780s on the LT1 cars.

forvicjr

Your right 650/750 same bore size,I ment 850. 650/750 1"11/16....850 1"3/4 been a while since this mod...
This is done to improve the flow restriction at the throttle blades and improve airflow on the 650 without losing the air velosity of the 750.Hence keeping crisp throttle response and supplying the air fuel the engine needs.