I am running a the motor in my sig and the car runs 12.60's on motor and the plugs are a perfect medium/dark gray color after every pass....perfect if i do say so myself. I'm unsure of the jetting, but it came off a 400 ci sbc.
friend just did the 302 carb conversion. he bought a 750 for it but it was 2 much carb. he has a vic jr intake and trick flow heads. he had to go to a speed demon 650. engine ran way better. i have a 750 on my 351w and it runs rich. i guess i should tear into the jetting a bit. if you were going to pull crazy rev'x like 8500 or so then i would say you need the added cfm but on a street car the 650 will be lots.
With all the stuff in my sig, I run a 570 vacuum secondary. I had the 670 and it was far too much. A lot of the folks who say 750's run "just fine" have probably "learned to accept that carbs idle crappy" and such and dont know what they're missing by going to a properly sized carb. The difference between the 670 and 570 is tremendous! Snappy plant you in your seat throttle response, kicks in on the first turnover when starting cold, great gas mileage plus all the power you need.
Get a 570, it wont choke off your motor that much really and you'll be too busy yelling weee while exploding out of every traffic light to notice you might have lost 5 hp at the very top. Remember, with a carbv, vacuum signal is EVERYTHING. A carb pulls gas out of the fuel bowl. Too big an opening and there is no velocity and everything sucks until you open the throttle up to let a lot of air in....even then it is inefficient.
Dont get a double pumper. All that accomplishes is bogging the motor. You do what any EFI person will do when you want to take off: you floor it. What happens? 2 sets of butterflies open and the air basically stalls and you engine falls flat on it's face then after a moment takes off.
Well, on a vacuum secondary, you snap the throttle open, only one of the throttle plates slams open and you keep enough velocity so the motor doesnt bog. Then with the added vacuum difference, the secondaries follow a split second later when _you actually can use them_.
My advice, holley's 570 street avenger. Solid carb. Very tunable.
Nobody has mentioned the "straw theory" so i'm gonna throw this in the mix. Maybe a lot of you will get the idea of why too much carb is just that, too much carb.
Think of how a straw works....you suck liquid up a tube and into your mouth right? Wrong. It's much more complicated than that. What you're doing is creating vaccum with your mouth. Everything on Earth is effected by atmospheric pressure (typically around 14.7lbs/sq. inch at sea level). In a glass of water, the liquid in the glass is being held down by atmospheric pressure, as well as the liquid that is inside the straw sitting dormant (before you suck the straw). When you do, you create an area of less pressure inside your mouth, which allows the liquid outside the straw to be forced up through it and into your mouth, which is the lower area of atmospheric pressure.
An engine operates on these same laws of atmospheric pressure. When the engine turns over, the pistons go into their intake stroke, which creates the same low pressure area inside the cylinders, which creates a vaccum....yada yada yada you guys know this part. The role that a carb plays in this, is that too big or too much cfm means that the engine does not create enough vaccum to draw in a sufficient amount of air to create an optimum mixture.
Consider this....you have 2 glasses to drink from. One glass has a straw in it that's 2 inches in diameter....the other has a straw that is 1/16th inch diameter. On the first straw, you're not going to be able to suck hard enough (create enough vaccum) to pull the fluid into the straw, and on the smaller straw you'll have plenty of suction but you won't get much for all your efforts (the same as not having enough carb).
There's a ton more to this but i just worked 12 hours and i'm pooped. What a lot of guys don't understand is that maybe their car ran "great" with a 750 or whatever, but with the proper size carb to match the rest of their combo, it WILL make more power and will run much better. When doing a carb setup, the carb should be the LAST thing you buy.
One more quick thing i'm going to add to this, is about air valocity. Air can be compressed (obviously, such as forced induction) but in a typical environment (natural aspiration) the air moves in accordance to it's surroundings. Let's focus on air moving through 2 different pipes.....one pipe is 2 feet wide and the other pipe is 3 inches wide. Both pipes have the same amount of pressure pushing the air through them, but the 3 inch pipe will cause the air to have to move much faster to be able to get through the pipe vs. the larger pipe. This is the problem with a carb that's too big, as well as having an intake that flows too much and heads with too large of intake valves (hell this even applies to having too large of an exhaust). If you create too much space for the air to move in and out of an engine (such as larger carb/intake/heads and so on) you end up with slower moving air. Everyone knows that the faster you can get air into an engine, the more power it makes, so once you find the right combo to flow the optimum amount of air, again you're gonna make more power than you will if everything is too big.