Holley 750 hp vacuum secondary or Holley Double pumper?

-
PG Duster

PG Duster

Well-Known Member
Joined
Oct 23, 2021
Messages
326
Reaction score
266
Location
California
I have a 1974 Duster, with 360 magnum 9.1 compression, EQ cylinder heads with 2.02” in take valves and port job, mild hydraulic roller cam, 727 trans with 3500 stall with 3:91 sure grip 8 3/4 diff. I have a Holley 750 Hp vacuum secondary Carburetor on a Edelbrock air gap rpm intake manifold. Would there be any advantages to buying a double pumper carburetor for better preformance or Is my combination good?

3BF691B4-9B75-42F5-ACF2-6F318F5217FB.jpeg
 
I personally like the vacuum secondaries on a car that sees a lot of street use, but either one should be fine. Whichever one you use, the most important thing is setting it up right.
 
Just be aware that the carb you have pictured does not have a choke for street driving/cold startup- probably not an issue since you're in Cali.
 
Double pumper would be my choice. But means you have a HP vacuum secondary, might as well run it.
 
The HP 750 vacuum secondary is set up right, and works great, but not having a choke on it and the air gap intake, during the few cold days in Cali it take a long time to warm up and stop sneezing through the carburetor. I was think about getting a 750 double pumper with a choke on it, but not sure if it’s worth the price to buy one. Is the vacuum secondary or the double pumper be the better choice for street strip? Would it be a performance enhancement that would be worth the cost over the carburetor I already have.
 
In your situation, I don't think you'd realize enough of a "performance enhancement" to justify the cost of a new carb. All you'd be gaining is the choke. I'd just live with it, especially if yours is dialed in already.
 
You have a very nice carb. With your gearing I would rather have a 750hp mechanical secondary but unless you can get a pretty penny for yours, I wouldn't make the change.
Let it warm up a few extra minutes on a cool morning. No choke needed IMO.
 
PG Duster said:
and stop sneezing through the carburetor.
Click to expand...
I would gradually, till just enough, richen the idle “circuitry” till that sneezing disappears. I’m running a 750 DP with no choke and have absolutely no snorting, coughing, sneezing issues on even the coldest mornings (SC mornings are likely colder than California) I’d bet your just a wee bit lean on things, and could sneak up on it till it disappeared without fouling :eek: up how it’s dialed in, raw gas etc burning eyes etc.
 
12many said:
I would gradually, till just enough, richen the idle “circuitry” till that sneezing disappears. I’m running a 750 DP with no choke and have absolutely no snorting, coughing, sneezing issues on even the coldest mornings (SC mornings are likely colder than California) I’d bet your just a wee bit lean on things, and could sneak up on it till it disappeared without fouling :eek: up how it’s dialed in, raw gas etc burning eyes etc.
Click to expand...
 
496 polara said:
You have a very nice carb. With your gearing I would rather have a 750hp mechanical secondary but unless you can get a pretty penny for yours, I wouldn't make the change.
Let it warm up a few extra minutes on a cool morning. No choke needed IMO.
Click to expand...
Yes, I agree that is a lot of money.
 
DP if you race a lot. The 4 corner idle adjustment and secondary nozzle are the benefits to swapping.
 
12many said:
I would gradually, till just enough, richen the idle “circuitry” till that sneezing disappears. I’m running a 750 DP with no choke and have absolutely no snorting, coughing, sneezing issues on even the coldest mornings (SC mornings are likely colder than California) I’d bet your just a wee bit lean on things, and could sneak up on it till it disappeared without fouling :eek: up how it’s dialed in, raw gas etc burning eyes etc.
Click to expand...
yep. I live where the snow flies and temps go below zero F, but I don't run a choke on any of my Holley's, except the one that had a manual choke hooked up in the van.
 
mderoy340 said:
DP if you race a lot. The 4 corner idle adjustment and secondary nozzle are the benefits to swapping.
Click to expand...
Good information, I will be going to the track about 7-8 times a year to bracket or grudge race. The rest will be street driving.
 
318willrun said:
yep. I live where the snow flies and temps go below zero F, but I don't run a choke on any of my Holley's, except the one that had a manual choke hooked up in the van.
Click to expand...
That’s colder than I will ever see in Cali, and you don’t use a choke. Wow!
 
PG Duster said:
I have a 1974 Duster, with 360 magnum 9.1 compression, EQ cylinder heads with 2.02” in take valves and port job, mild hydraulic roller cam, 727 trans with 3500 stall with 3:91 sure grip 8 3/4 diff. I have a Holley 750 Hp vacuum secondary Carburetor on a Edelbrock air gap rpm intake manifold. Would there be any advantages to buying a double pumper carburetor for better preformance or Is my combination good?

View attachment 1715836519
Click to expand...

Im a total DP’ee fan!
 
PG Duster said:
That’s colder than I will ever see in Cali, and you don’t use a choke. Wow!
Click to expand...
Here's a story, and I'm not chokin' !! Couple years ago, my wife went out around midnight in the middle of a winter storm, 17 degrees, swept about 9 inches of snow off the Ramcharger, started it, broke open her own trail for miles (plows were called off at this time) to come and get me when my Ramcharger tranny line let loose and I lost my transmission fluid. She did it all by herself without a choke :)
 
318willrun said:
Here's a story, and I'm not chokin' !! Couple years ago, my wife went out around midnight in the middle of a winter storm, 17 degrees, swept about 9 inches of snow off the Ramcharger, started it, broke open her own trail for miles (plows were called off at this time) to come and get me when my Ramcharger tranny line let loose and I lost my transmission fluid. She did it all by herself without a choke :)
Click to expand...
NICE!
 
318willrun said:
Here's a story, and I'm not chokin' !! Couple years ago, my wife went out around midnight in the middle of a winter storm, 17 degrees, swept about 9 inches of snow off the Ramcharger, started it, broke open her own trail for miles (plows were called off at this time) to come and get me when my Ramcharger tranny line let loose and I lost my transmission fluid. She did it all by herself without a choke :)
Click to expand...
never mind the choke, I need a wife like that! ;)
 
Mine came from the middle of Long Island to southern Delaware to save my ***. Yea! She even bought her own Trick Flow heads & rockers.
 
Bear in mind that if you richen up the idle [ post #8 ] so that without a choke the carb doesn't 'sneeze' on cold start ups, it will be rich at idle at operating temperature.
 
Bewy said:
Bear in mind that if you richen up the idle [ post #8 ] so that without a choke the carb doesn't 'sneeze' on cold start ups, it will be rich at idle at operating temperature.
Click to expand...
At first read you make it sound like that’s a bad thing or a negative. But you’re right. Considering that we don’t know just how lean his idle circuitry might be, but it is, obviously he needs to go a tad richer to eliminate that, or just ignore it. Just a cold morning issue.

Me? I set my idle circuits for best quality and running, not by trying to achieve any supposed ideal number on a A/F ratio gauge (even though I run one) or plug color. My reading at idle is about 13.5+/- iirc but not because that’s what I want, that’s where it’s at after tweaking for vacuum, idle quality etc.

Had my first real taste of hard cold starting and warm up idle sneezing and buck snorting on an old Triumph Bonneville 750 with Amal’s. Only took slight richening of the air screws to eliminate that malady.
 
Thank you guys, it sound like the HP 750 vacuum secondary is great for street strip. I need to Make it a little richer for the cold start. Will the mechanical secondary et or have the same trap speed at the drags as a vacuum secondary carb?
 
Last edited:
PG Duster said:
Thank you guys, it sound like the HP 750 vacuum secondary is great for street strip. I need to Make it a little richer for the cold start.
Click to expand...
No, the choke does that for you.

Will the mechanical secondary et or have the same trap speed at the drags as a vacuum secondary carb?
Click to expand...
No, it should be possible to be quicker with the DP. A minor amount, but yet there. The thing being is you can open up the secondaries quicker. One of the things about a DP is you can open them up to early. This you’ll learn by feel and/or drag strip testing.
 
You must log in or register to reply here.
-

Back
Top