I was running a 650 edelbrock on my engine and my car ran 12.6 in the 1/4 mile but I bogged off the line and couldn't spin the tires. I Put a Holley 750 DP with a Proform main body on it that should flow well over 800cfm, now the problem is that I spin the tires off the start and don't bog but now I have no high RPM power and the car only runs 13.1 with the Holley. This really boggles my mind because I thought the larger carb would give me the exact opposite results. My first runs with the 750 were with 72/82 jets and I ran 13.5, then I put in 76/86 jets and it ran 13.1. All my 60' times suck and are in the 2.0-2.1 range. Should I try even bigger jets? It feels like I lost all the high end power when I switched carbs. Also my mph dropped from 112 mph with the 650 to 108 with the 750.
Please give me some advice on what I should try.
Thanks.

 

Did you put the air bleeds in the correct place?

 

The air bleeds on the Proform mainbody are the same way as it came new.

 

what are your plugs telling you?

 

If you are losing MPH, you are too rich. You are going the wrong way in your Holley tuning. Put in less jet. 76/86 is wayyy too rich for what you are doing. I ran 11.70's @ 115+ with the stock 72 front, and 80 jet rear, and that was in a 3310 vacuum secondary 750. Go back to the 72 in the front, and put a 78 in the back. See how that works for you. 86's are way too rich unless you're running without the power valve.

Are you running a sticky tire at the track? If not, you'd better get a set of DOT or slicks. Trying to tune the car with jetting and inconsistent launches on street tires will be a nightmare.

 

Try 73 primary 81 seconday assuming there is no power valve on the secondary side.

What are your air bleed sizes? The outside is your idle bleed. A 750HP has .075 idle bleeds, and 0.036 high speed bleeds. A smaller high speed bleed will act like a larger main jet and start the main circuit sooner. The opposite is also true.

This should be a good baseline that mimicks a stock 750HP jetting. Though your metering blocks are different this should get you close. Jet for MPH at the track and make sure your timimg is very close before you start working too hard. Don't spread the front to back jetting more than 8 sizes. Chances are you'll end up being around a 6 size spread.

 

Thanks for the info, Sounds like I have a lot of tuning to do. I just got in from the garage and I fixed my linkage problem, my linkage was too short and wasn't opening the butterflies all the way. Now that I can get full throttle it is bogging a little if I gun it off idle. I checked my plugs though and they look normal, not white but not fouled with fuel on them. I am going to try different jets and see what size my air bleeds are.

 

Boy does that sound familiar. I had the same in another vehicle, put the DP on and lost performance. Went back to examine what could be going on and came up with a number of ideas. Went back to the Edelbrock and never looked back.
Are you running vacuum adavance or pure mechanical? Do you have a vacuum gauge on the intake? What is the initial timing? The Edelbrock should be able to spin the tires with no problem, but depending on you vacuum signal to the carb at different RPMs you may need some tuning. Holleys don't offer the range of tuning here that the Ed will, Holleys have their place, I have several.

 

All of my carbs have mechanical secondaries. My timing is good, 20 degrees initial, 20 mechanical advance. I think that my carb gasket between the main body and the butterfly plate is leaking and sucking in alot of air. I am going to tear it apart and make sure all the gaskets are good and I am going to try different jet combos. I am also going to get new air bleed screws and try that. I am surprised that the DP carb would make such a negative impact on the top end power.

 

Make sure that the accellerator pump check valve needles are in place under the screws. Its not the carburetor itself, you just haven't nailed the tuneup down yet.

 

I drove it today after making sure there weren't any leaks and it drove like crap, I rechecked the plugs and now it looks like its running rich. I swapped my large accelerator pump for a stock one and tommorrow I will set the jets lower but I tried the jets low before and it ran real slow. What else should I check for besides the jetting and air bleed screws? Could the squirters be too large? Could a 750 DP with a proform main body be too big of a carb for a 350 with a 236/242 roller cam, trick flow heads, etc.?
I appreciate all the advice and help you guys are giving.
Thanks.

 

Check needle and seat o-rings. Make sure you don't have nozzle drip with the engine sitting there idling.
Stock 750HP squirters are .031 pri/sec. After the pump shot is used up the circuit doesn't effect wot mixture unless your check valve needles are missing, then you get massive fuel pullover at the upper rpm. What metering blocks are you using? Are they from a 4779 750 double pumper?

 

What power valves are you running, sounds like they mat be opening too soon.

 

Holleys are very tunable carbs, *if* you know what you're doing with them. The Edelbrocks are all vacuum seconday carbs, which makes them great on the street. Holley also makes a number of vacuum secondary carbs, like the venerable 3310 750, which work fantastic once properly set-up.

A guy I know had a 1980 Z28, 355ci with the Performer RPM cam & intake, with ported camel-hump heads, 3000 stall and 4.10 gear. Started off with the Edellbrock 750 carb, and another bud loaned him a Holley 750 with the choke horn milled off and he picked up 2 tenths and 2+ mph at the track.

Did you make sure to get the Proform main body for the 4779 and not the 3310?

 

I would look more into your accelerator pump, put the 50cc pump in the rear and front and tune it with diff cams that come with the kit. bogging off the line means too much pump. you need to tune the pump to not run out like it is before the enrichment system kicks in..Accelerator pump cams come in various sizes and are color coded and number coded by Holley. The cams have different shaped ramps that the arm from the accelerator pump rides on. By changing the size and shape of the arc on the cam, the pump shot can be tailored to start early or later as you go from off idle to full throttle. Changing the cams can have an effect on the way a vehicle leaves the start line in a drag race. If you leave the line off idle or at a higher RPM (while foot braking or when using a tranny brake or when using a clutch with a manual transmission) experimenting with the pump cams can help. There is no set rule for use, you just have to experiment with the different cams and the different cam positioning holes in the throttle linkage of the carb. Holley sells individual cams or you can buy their kit which includes an assortment of cams to choose from. Pump shooters are another area of experimentation. Holley carbs come with a standard shooter size which differs by carb list#. If you are experiencing a bog or hesitation off idle, you can try a larger, higher # shooter size. The bog or hesitation may be caused by a momentary lean condition when the carb goes from the idle throttle position to the main metering system. The shooters help richen this momentary condition and eliminate the stumble. Keep going up in shooter size until a puff of black smoke comes out the exhaust, then go back one or two sizes.

Playing with the shooter sizes is particularly helpful, when you have an intake with a large plenum area, such as a large open plenum or a tunnel ram. Keep in mind that as you increase the shooter size, you may also need a "hollow" screw to hold the shooters in the carb. At shooter sizes over .039, Holley recommends that you use the "hollow" screw (PN-26-12) which allows more fuel to flow to the shooters.