I need some help tuning my holley 750 dp. The engine hesitates bad, almost dies when I hit the throttle fairly quickly. I have read numerous threads about it but nothing seems to work for me. I'm pretty sure I am experiencing the "carb too big" problem,as it did the same thing with an edel. 750. Floats are set right. Checked for vac leaks. Idle mixture was set properly. Total timing is about 36 degrees(before vacuum advance). Accelerator pump arm has very little if any slack. I have tried a 45 power valve(from factory), a 65, a 85, and now a 10.5. I think it got even worse with the 10.5. I changed the front squirter to a 37. That helped a little in combination with richening the idle mixture(when running the 45 pv.), but the increased idle mix fowled my plugs quickly. I pretty much tried everything except jetting. I even installed a 4 hole 1" spacer(didn't help). This is about to drive me crazy, as I have put a lot of money in the car and can't even drive it! Any help will be GREATLY appreciated.

Here is a list of my current setup.

MOTOR.........
350 4 bolt, TFS 195 aluminum 62cc heads(2.02, 1.60 valves)
Holley 750 street hp double pump, factory jets(75 front 80 sec.), 37 primary discharge nozzle, 28 sec.
Edelbrock RPM airgap intake
MSD 6AL, Accel HEI
hydraulic .480/.494 lift, 228/234 dur. @ .050 cam (110 lobe sep.)

TRANS..........
700R, shift kit, 3000 stall

 

My friend as a edelbrock 600cfm on is 350 trans am and is car works great better than mine trans am with a 305 stock.

 

Sounds like an accelerator pump issue. Have you experimented with different pump cam lobes? What colour lobe is on there at the moment?

The 10.5 power valve shouldn't make things worse unless it's already open when you put your foot down - I.E. it's opening under cruise conditions. How much vacuum do you have at idle, as you 'tip in' and at cruise?

Jon

 

I went though this awhile back. Finally rejetted it and it fixed it right up.
Believe it or not, you have to put BIGGER jets in it if the carbs too big. I know, I didn't believe it either but it's true.

Dan

 

I'm not sure what color cam lobe. I'll have to check. It is whatever came on it. Last time I checked the vac. reading I believe it was about 12.5 or 13 at idle in gear. Thanks for the replies. I'm not gonna give up on it. Maybe Jon is right. I might need to play around with the acc. pump.

 

That's a funky squirter stagger it's running. Try a couple of 31 tube type squirters instead. I ran a 750 on my old 305 and didn't experience the "carb too big" bog you're talking about, there's another issue here.

 

OK, I know that I will take some flack for this, but, if you are not racing this car, go with an Edelbrock 600. Bolt it on and go. Minor tuning, fuel mileage, and performance. This is my experience on several engines. Enjoy the driving experience of your car! My 2 cents. Randy

 

I second the accelerator pump issue. May need a 50cc pump conversion on the front. Is it popping back through the carb when it stalls? Typically that points at a lean condition (ask me how i know lol). I think the 6.5 power valve should work fine, my buds race car is still running on a 6.5 with 210cc heads and a nx286 cam. I would pick up a pump cam assortment first, give those a shot. The 50cc conversion is a bit pricey but its worth it.

Edit: To Randy
His engine combo would without a doubt run hella lean with a 600 edelbrock, if anything a 750 would be in order, but you can tune a holley to get everybit of mpg hat an edelbrock does via wideband, and have better performance to boot!

 

You cant just throw whatever power valve in the carb and hope it works. You need to see how much vacuum your car is pulling and then select a PV accordingly.

 

I'm not just throwing any power valves in it. I took a vacuum reading at idle in gear(12.5). I then put a 6.5 in it which is about half, which is what a holley performance tech advised. When that didn't help I was advised from a local speed shop to take the number from the vac. reading and subtract 2. That would be about the max size without inrichening too soon, and that is when I worked up to a 10.5

 

Thanx. I believe I'm gonna try the pump cam idea.

 

I expect that you already know this but as a side note to power valve selection, deviding by two or subtracting 2 is still estimation and guess work. If you can get a hose long enough to bring the Vac. gauge into the cockpit you can see where your lowest vacuum occurs.

For sure, when your foot hits the floor the vac. is going to bottom out and you want the power valve to react as quickly as possible. But if during moderate acceleration as you transition form one cruise speed to another the vac. drops below the number on the power valve (10.5") your valve will be opening needlessly - causing a too-rich condition.

Jon

 

28 primary squirter and 31 secondary. Blue primary pump cam, orange secondary. I wouldnt go with the 50cc accelerator pumps. Jetting sounds high but then again i am used to the high altitude here.

Sounds like you've got too big of a pump shot in the primarys. Do you have the secondary power valve blocked off?

 

Well in your first post you said you used 4 different power valves. If you know which one your suppose to use stick with it and look for the problem somewhere else.

 

orange pump cams

 

orange pump cams.