For my two cents worth...750 is too big for a 350 but you can live with it. It all depends on what you want to do with the car. If you're going to race it or a driver. A 750 can be tuned for a 350 but you need to know what AFR you're getting when making changes. It's difficult to read plugs now a days...so you need an AFR guage or take it to a dyno tune session which can be costly. I put an AFR gauge on my car because I'm always looking out for any lean conditions.

 

750 vs 650 is the airflow rating, not the fuel. Changing jets adjusts the fuel.

You jet to get the air/fuel mixture correct, but it will never compensate for the wrong size carb.

Now for size, a 750 is large for a 350 if it is a mechanical secondary carb... unless is is a high rpm motor with steep gears.

If it is an Edelbrock carb, or a Vac secondary Holley, it is fine for your 350. Just jet it correctly for your application.

 

Qjets are 750's and they're on a millions of vehicles.
You just have to tune it. From the sounds of it, smaller jets are a definite for you, just to start.
But I agree that a vacuum secondary 650 would be more in line for a street driven car.

 

Didn't GM put 780's on 302's ?

I think if it's a street/strip car a vacuum secondary would be OK...but for regular street duty... a 600-650 would be better with crisper responce. (this also depends on auto or manual tranny and rear gears)

 

Didn't GM put 780's on 302's ?
I think if it's a street/strip car a vacuum secondary would be OK...but for regular street duty... a 600-650 would be better with crisper responce. (this also depends on auto or manual tranny and rear gears)

Yes, but only so it would be SCCA-legal for use on the Trans-Am cars that spent their life at WOT from 4000-8000 rpm all the time. A 600 Holley is a much better match for the 302 for street manners and driveability, with crisper throttle response due to the sharper metering signal, but they had to use the 780 on the production car to make it race-legal. I ran a 600 VS on mine while my 4053 was being restored, and the difference was very noticeable.

:beers:

 

It seems to be running too rich - poor mileage, gassy exhaust, plugs tend to get carbon down in the insulator area

As has been stated many times carburettors work effient in narrow rpm ranges and it at the top of the rating....this applies to street engines ...
Track engines its more a calultion of the potentual HP in the amount of air and fuel supllied under full loads....so they can be overcarbed and be powewr efficient ..that is a rich mixture
OK back to a street cruiser engine
So carnd are inefficient outside their top cfm range...so are cams, so are headers etc
IOf u want efficiencty right thru the rpm range and power range...cruise to WOT go injection.
Your 750 on a 350....ball park , a 350 cruiser engine 1000 rpms =100 cfm 2000= 200 cfm......6000 =6000 cfm 7500 =750 cfm....see where this is going to...do u cruise at 60mph and 7500 cfm?
Do u cruise and use 95% of the time in the rpm range of your can? no?

The factory because of this even put 2 barrel carbs on SBs
Now reducing the carb size also increases quite dramatically the bottom end throttle response...this makes it a little looser in the rear end ....
I run a 465cfm carb at 60 mph I have 2000 rpms
I start to lean in the mid 4000 rpm range...get to the low 5000 rpms leanenough not to run the engine for long period 3 /5 mins ....but that works out to cruising around 150 mph

The secret to engine design, is drop ego, drop bigger is better, then build the engine to the rpm range the cars final gear ratios is going to work in....carb , cam , headers, heads etc.

This is how a full on track engine is designed and built ...and your chainsaw and lawnmower engine.


Yesa there arespread bores, run on small primaries , largesecondaries, and yes this technology improves big carb efficincy
A 350 ci a 600 or 650 spread bore will work well.....slight improvement over a 600 650 square bore for street
But spread bore technology still doesnt get close to ECU and injection.

 

I have an Edelbrock Performer 750 CFM Carb on my '68 with a 350 Engine with an Edelbrock Performer RPM Manifold, a mild Cam, and Headers. It seems to be running too rich - poor mileage, gassy exhaust, plugs tend to get carbon down in the insulator area. Many people are telling me a 750 CFM is too big, and I should get a smaller Carb. I am wondering if putting smaller jets and rods in a 750 will accomplish the same thing as getting a 650 CFM. I don't understand how size matters! Other than the maximum amount of air that can pass through the carb under WOT. Can anyone help me understand the differences, and give recommendations. I have already purchased the Edelbrock jet and rod kit which I used to set the 750 CFM carb for my altitude, so I have more jets and rods available. Thanks.

Your AFB only flows 750 cfm on an engine that is big enough and turning sufficent RPM to open it fully.
Same as a 750 VS Holley.
The primaries flow about 325 cfm, the AFB and other VS carbs only add just enough secondary airflow as dictated by an air valve or a spring and diaphram.

You need to get your jetting/metering rod mixtures properly set. As far as the idle, make sure your transition slots are not over exposed.

The Q-jet is 750 cfm
The much recommended 3310 is 750 cfm
The 302 had a 780 cfm VS Holley as did the 350 Z28.

Would the 625 AFB be a better choice if buying new for a mild 350 that spins up to 6,000 RPM at its peak,,,, Yes.
Can you tune the 750 AFB to work? Yes.

 

The 302 had a 780 cfm VS Holley as did the 350 Z28.

One does have to take into account this was because of requirements to race the car for which it was designed....inclusing the under 5L requirements.
Basically a track car made to run on the street to get enough sales to quailfy.
And it did win on the track.

 

According to the specs, the Eddy 600 has 1+7/16" primaries with 1+3/16" venturis, whereas the 750 has 1+11/16" primaries with 1+7/16" venturis. Sooo (physically speaking) if you set the jets, rods, pump, etc. on your 750 the same as the 600 -- they'd still be different.

I think the (underlying) question that you're asking is -- what are you giving up with a jetted down 750 compared to a 600?

Basically, the larger primaries don't work as well at low airflows. This means you'll have less "crisp" throttle response at lower revs and might have issues getting the mixture right at low speeds since the fuel won't atomize/mix as well. For sure, a 600 would be more "ideal" based on your specs, but you should be able to get a 750 to work reasonably well.

Aside from the jetting and idle screws, be sure to check the float height and make sure the step-up piston/springs are working correctly...