306 Carb help

[306mstang]

I have a 306 sbf in my 87 notch, it is about 12:1 compression, not sure of what heads are on the motor but they have big valves and high flow ports, trick flow stage 2 cam, victor junior air gap intake and a holley 4150 750hp double pumper, mech secondaries. I have been using 100 octane aviation gas. Ignition is msd 6al, I had my ignition timing set to 18 initial, 36 total with the light springs for a steep rate of advance.
No matter what i do with the carb, tuning wise as far as pump cams and pump nozzles, the car always bogs or has a flat spot off the line. Being that the car has 4:30 gears in it and that the cars fairly light i think it should fly but i keep having this problem. It also seems spuddery while driving. Does anyone think a 750 is a little too big, I bought the motor with this carb on it and always thought 750's were for bigger motors? or am i just not getting enough fuel into this tiny motor? Someone told me put the 50cc accelerator pump diaphram on it because the carbs not squirting enough fuel in the beginning. any advice will be greatly appreciated. thanks

 

[Rick 91GT]

I think the 750 is too large...

your issue could be the lack of jet extenstions in the rear, on hard launch you are pulling the fuel away....very common. Youll need extentions and a new float...under $40.

What jets are you running in the carb? Typically if the carb is too large you have large jets to make the carb pull hard enough on it.-

 

[12Pack]

I had a 750 on my 306, it was wayyy to big, i downsized to 600

 

[rd]

I have a lot less engine in my 82, it's a flattop rebuild with iron GT 40 heads. It will spin to 6000 rpm anytime with just a 600 cfm vac secondary Holley 80457. I went up a couple jet sizes and have a light spring in the secondary diaphram. It lauches nicely, and runs clean all the way. Best of 12.90 or so.

I was thinking of going up to a 670 Avenger, but not much more. I was thinking that would also work on a 331 when I get there.

For the carb you have, a bigger pump shot, or a cam that spreads it out might help to get the engine into the range where the jets kick in. Might want to try a quicker opening power valve. Have you tried less timing? That might help it rev quicker.

 

[cammed89lx]

go with 650 carb i have a somewhat close set up. and i run a 650 with gets the job done.