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Discussion Starter · #1 ·
So Here's my question, i think this is the right place to post.

I recently rebuilt my motor and did everything but the carb. My car runs rich at idle, and when i go to punch the throttle it hesitates (shooter/squirter?) before it takes off. the car make loads of horsepower above 3k RPM. Here is my current setup:

396/375 bored .90 over
Comp Cams Xtreme energy roller tappet Duration 274/280 lift .639/.646
Holley 750 Double Pumper
11:1 Domed Total Seal
Weind Stealth Dual Plane intake
Square Port L-78 Hi-Performance 1968 Heads
Solid Lift, Roller everything
mechanical advance MSD PRO billet dist

This car make LOADS OF POWER above 3k RPM (square port heads). The timing i believe is good, i would have to re-check the car to tell you what it is. the stock 396/375 i believe had an 825 or 850 cfm carb on it. i want a larger one with more fuel, more airflow.

Going down the road sometimes at about 2-3k RPM the car just loads up with fuel and floods the engine and dies.
Also, the car doesn't run good when its hot outside, a noticeable difference when its cool outside.

Can you guys suggest a good carb and its size? do you guys have a choke?

2,183 Posts
Personally, I would stick with what you have. If it's set up right, that thing will scream. You told me about your engine, what about the carb? Are the power valves still there? If not, PUT THEM BACK!! The biggest flub on Holleys I get around here is, everyone thinks they have to plug the power valve. You can, ....if your running two carbs on a blower or tunnel ram set up. Otherwise put them back in.

Ball park jetting should be around 68 primary and 72 secondary. That might be a little on the lean side, but start there. Make sure the accelerator pump arm is adjusted properly. The spring loaded bolt should be just resting the pump arm and not applying any pressure at idle. Put the stock power valve in.

Now try this, start it up and let it idle. put one hand on the accelerator pump arm and one on the throttle. Simultainiously dump the throttle and stab the accelerator pump at the same time. Still have a flat spot? I'll bet you don't. Try using a more aggressive accelerator pump cam. You could also try a bigger squirter. Stock should be about a 25. Try to bump it up to about a 28. Anything over 30 requires a hollow screw to hold it down, so you don't lose fuel flow.

Holley's are simple to figure out. If your running down the road and it loads up, your main jets are too big. You can also have a lean problem off idle that creates a flat spot. That's why they have power valves, adjustable accel pumps, and squirters. for the transition to w.o.t., and main jets for once you are there.

Another test for mixture. Start it up and let it warm up. Bring a plug wrench with you. Take it out for a blast. Stand on it and go through all the gears. AS SOON AS you're up to speed, shut it off and coast over to the side of the road. Pull a plug out and look at it. Tan is perfect, black is rich, and white is lean. You CAN NOT drive back to your driveway to get an accurate reading. this reading is for the mainjets. if you back out of it and go home, you'll be reading the accel pump, power valve and pump cam, too. That will give you a base line for the mainjets. As far as everything else, from a dead idle, you should be able to dump the throttle in neutral and have the engine rev immediately. If you have a flat spot at all, you missed something. Hope this helps. If you were closer, I'd tune it for you. A 750 DP is fine. You don't need another carb, you just need to tune that one. I ran a 750 Vac. Sec. on my old Dually with a 468 and 9.5 comp. Throttle response was like lightning and I could burn all four tires off the truck. I have a 10:1 454 that I'm only running a 750 on and it's just plain mean, also.
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