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Holley 750

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Garys1969RR

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Running a 4779 Holley 750 Dbl Pumper on my 440. Seems to run rich, plus has a stumble right off idle. Probably in the transition circuit. Plus I can hear a whistling sound at times when I'm at part throttle. So I'm suspecting a vacuum leak. Runs fine at all other openings. It has a Performer intake, CH-4B old school. Thinking it may have a gasket to carb sealing problem, or ???? Taking the carb off tomorrow to see what's up. Would like to go to a Performer RPM with an 800 CFM AVS carb. Anyone have an AVS for sale/trade? OK Thanks
 
The double pumper carburetors are not a good choice for a street car. They do dump too much fuel. An Air/Fuel gauge would confirm that. Now, a 750 with vacuum secondaries is a good carburetor for a 440. I have had them on a few, even a few mild 360 engines. The vacuum secondaries only deliver what the engines need...the Double pumper dumps gas whether the demand is there or not.
AVOID the Edelbrock 800 if you have anything bigger than a stock cam!
 
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The 4779, IMO, is about the best value minded performance carb out there. I've run them on 340s, 383s, and 440s on factory intakes and aftermarket aluminum dual planes, from nearly stock to mild street engines.

What you are describing is some sort of a problem in the idle circuit, or as you said, in the transition. Despite popular feed back on forums, I have never found the squirter to be the issue on a light throttle off idle hesitation. Its usually timing, or lean idle condition caused buy a vacuum leak, low fuel level, dirt in the idle circuit, idle screws adjusted too lean, or a combination of some of these. If you get into the throttle pretty hard and it stumble pretty good, then maybe the squirter.

The Performer RPM is a well love intake as well. But unless you have a pretty decent 440 in the 450 hp range (i.e. a solid 12 second street car), you really won't see much different in but a little in peak, and it will not be quite as good under around town under 3000.
 
Thanks! It has an A/F GAUGE, and showing 11 to 1. Sometimes 12 to 1. So will pull it off tomorrow, and work it over. Seems to be some play in the primary throttle shafts. May need new bushings, and could be a source of a vacuum /air leak.
 
i've had several double pumpers that worked great for me I liked the mechanical secondaries if you set them up right could always get a little more out of the motor with those, I also had one that had a sloppy butterfly shaft which caused a vacumn leak. the problem with most of them is that there jetted to rich or a float level. I'd keep it. jmo.
 
Yes I think I will keep it, and try to get it working properly. I think its jetted too rich, at 72 primary. I'm at 5600 feet. Stock is 70 pri /73 sec . That combined with the vac leak is messing it up.
 
What is the jet size reduction for altitude? Is it 1 step per 1000 feet? Or maybe it's 1 step per 2000 feet. I think that's more like it.
 
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Or you have a vacuum leak to power valve which will cause it to run rich
If metering block is warped PM and I will tell you how to straighten
 
Man, I've run DP's on most all of my stock street junk and had great results with them.....but yeah, they do not give the greatest in town mileage since you're on and off the throttle a lot but will do pretty good on the highway if you can learn to tune and drive with them.
 
Start it up spray some carb cleaner around the base then spray around butterfly shaft areas while putting forse up and down any leaks will change your rpms. If good check plug color should be black if running 11 to 1. Then buy a Holley book and a rebuild kit then it's new and clean I always start with the primary jets 2 sizes down is a good start while having your air screws about 1 and a half turns out. Stay out of the secondaries. Ive never used a af gauge just checked plugs for a nice tan color. If your can get the air screws to run good within a half turn within the 1 and a half and burn tan your close if it still black go smaller jet again. After you get that I would check your timing. Then the fun part is the second aries. Same thing just activate the secondaries alot check plugs again. Get that close then buy new plugs and adjust your air screws on each side by taking a plug out if each side.of the motor. A six pack at least is required before starting and I don't mean holley. That's how I always do it I'm sure there are different methods but that worked for me for many years. Good luck
 
I believe that most people who say a DP is not good for the street have never had a DP.
 
forphorty said:
I believe that most people who say a DP is not good for the street have never had a DP.
Click to expand...
I believe I'd trust Rick Ehrenberg before I'd trust a guy running a double pumper on a stock engine with an automatic transmission.
 
The idea that double pumper carbs “dump” fuel is nonsense.

The only difference is the secondary accelerator pump shot when the secondary shaft moves.

Fuel is drawn through the boosters via a Venturi effect.
This goes for vac secondary and mech secondary carbs.
 
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Put a 500 CFM Carter AFB on there today, that I had recently rebuilt. Wow what a difference! Fired up immediately! No flat spot, and starts up every time, great little carb. A/F ratio around 14 to 1 at idle. On the 750, it looks like the primary power valve is bad. Won't hold vacuum, or stay closed. That could account for the vacuum leak and over rich condition. I'll keep this AFB on there for awhile, until I decide to hook up the Paxton blower. This IS a stock low compression RV engine, with a blower, that's why they had put a 750 DBL pumper on it. Just trying to get the baseline tuning done, so I can use the blower later on. Just going to do some street cruising for now.
 
The 750 DP that came with my car had a decent sized vacuum leak at the shafts. This was confirmed spraying brake clean near the base of the carb with the red straw to pinpoint application. I replaced the complete carb base with a new one and it cleaned up my idle perfectly.
 
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