Holley HP carb 750 race no choke

[jerrycars41]

Hi; Just switched from edelbrock 650 elec choke carb to Holley racing carb 750 HP double pumper no choke mechanical secondaries. My edelbrock gave me 12 mile per gallon. My Holley runs starts idles great. Only 4 mile per gallon, unacceptable!!!. Any one have any suggestions. There are no leaks as it was checked from tank to carb.
This carb is on a 383 stroker in a monte carlo s/s 85 with 700 R-4 with 2500 stall solid cam and lifters with edelbrock air gap and alum fast burn heads.3:73 posi rear 18"tires Car is nor raced and driven on street.burn 93 Sunoco in it
Thanks for any and all help.
Jerry

 

[pontiacgp]

a 750 is too large for the engine and an HP and not meant for street use. We use a Holley HP 500 on our 604 crate that's running over 6000 rpm for 100 laps.

 

[BuWaGone]

That 750 is too big for the street but I understand that many of us can't just drop the $$ for another carb. I'm not sure how experienced with carb tuning you are but things I'd look at are squirter size, jet down and change your power valve it accordance to you cruise vacuum. Get S-A Holley rebuilding and modifying book and read the whole book til you understand.

 

[Bar50]

You might try having an air gap between the accellerator pump and the pump arm/lever. You see, every movement of the throttle dumps fuel from both pumps. Try slower, milder, smaller pump cams. Heck, try removing the rear pump lever/arm for driving (its a test).

Holleys, HP for sure are race carbs. Might try and borrow a 650-750 vacuum secondary froma buddy to tryout. The Edelbrocks are driver carbs, but, do not like rough idle, neither do most Q-jets, but, the offer better mileage. You can tru jettingdown your carb or maybe even the power valve if loosening up on the pumps does not help....

As far as cfm ratings go, thats so much...well...my 496 with square ports and a solid roller is running an 850. You can run a 750 on a mild motor, will work much better with a dual plane intake. Bigger carbs make for soft (slower throttle response) that require some tuning. A single plane intake, such as a Victor Jr will make a smaller carb feel and work like a bigger carb due to the open plenum. A restrictive, torque building dual plane will be more responsive, and is more sensitive to the 'right' carb size. There is no magic bullet carb.

A 383 and a 700R4 would lead me to gather that you enjoy driving your car, run your HP on the weekends and for Test & Tune with some slicks. Find a used 650 or 750 Edelbrock and buy the tuning kits for it and go from there. I don't know what solid cam you are running (obviously for performance), my last 5.7 rod, 9.75-1 CR, 383 has Vortec heads, fake air gap intake and a 625 Demon with the Xtreme 268 cam....it is awesome. It would be faster with a 270-280 cam and a Victor Jr with a 750....but its in a daily driver (no 700R4 yet).

 

[BuWaGone]

How big is you cam? Your power valve may be causing your carb to run rich all the time. Hook up a vacuum gauge and observe your vacuum at various steady cruising speeds. Take the lowest vacuum reading and get a power valve about 2 in(Hg) lower. Try a squirter a couple sizes smaller. Get a wideband and jet for the best AFR at steady cruising.

 

[thrasher]

If you really put the "HP" model on there that probably wasn't the greatest move. The richer idle/transition circuit on the HP model is most likely the culprit.
But then again have you dialed in the power valves and the jetting?

If you're not racing it a Holley 750vs with a secondary metering block would serve you better on the street.
If it was me I wouldn't go any smaller than a Holley 750 vacuum secondary.

Having said that a Holley 4777 650DP will bring a hell of a lot of snap to that engine down low.
Try one and you'll see what I mean :shock:

I would even entertain trying a Holley 600 Double Pumper but in no way would I think about trying a Holley 600vs. That is how much I hate them!
They (600vs) might not show that bad on a dyno but the small cfm double pumper is so controllable and fun on the street that it's a no brainer to me.

Some of you are gunna think I'm nuts here but ...
Depending on how much cam you have you might also try a Holley 750 DP list# 9379 with Annular Boosters.
Annular boosters give one hell of a kick down low.

 

[jschevys1971]

Soinds way rich.....
Blown power valve????

 

[crotchss]

JW did you check mixture screws to make sure they are set correctly? Do you have vacuum advance hooked up to the distributor? Also from my experience most Holley carbs(except the Street Avengers), but especially the HP's are jetted pretty rich out of the box. Alot of this could be attributed to the fact that where I'm from the altitude is around 2200-2500 ft. Most carbs are jetted at sea level, hence I end up taking out at least a few jet sizes most of the time. BTW what are your cam specs?