I'd like to add that with an automatic you should generally run a vaccum secondary carb.
A double pumper would give more throttle response but is more critical to being just the right size, a little too big on a DP carb and you can get a bog.
If running low gears like 4.10 or lower, and a four speed on a 350 with a healthy cam, the 750 should be an ideal size for best performance.
Get the car in high gear and floored at about 5000 rpm and try letting off the throttle just the tinyest bit and see if the car pulls better. If it does, you need a smaller carb.
I had an 800 DP on a LTI 350 with 4.56 gears and headers, ported angle plug iron heads, single plane manifold, etc. I noticed it went better when I backed off in high gear. When I put in a hotter cam, it took all the carb had then. I then swapped in a 750 DP a friend had and noticed a small drop in power and first gear "jump". He had a Z/28 with stock solid lifter cam and 4.10 gears, and headers. He tried my 800 and it was too much for his car. He ran best with the 750 DP.
Based on this experience, I bet there are a lot of factory stock Z/28's with 780's that never have their secondaries open fully! I'd like to rig up an indicator light on a 302 Z/28 carb and see if I'm right.
The 780 has the same venturi size as the 800 I had. and if it was too big for my 350 It probably would have been too big for a 302.
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