Recently given a Holley double pumper 750 and i was wondering if there is step by step guide on turning it down to a 600. I've searched a bit and came up with nothing, any help would be much appreciated.

 

Step 1: List the 750 for sale on the TGO Classifieds, eBay, etc.

Step 2: Look for a 600 for sale on the TGO Classifieds, eBay, etc.

 

Put it in a box and ship it to me. I'll promply ship you back a 600 holley. Do you want a vacuum sec or a DP version?

 

Ya can't turn it or tune it down to a 600.

Why would you want to?

Tell me the "list number" stamped on the carb's Choke air horn and some basic info on your car/ engine specs and I'll tell you how to set it up to run like a 600 holley, even better.

Once fine tuned to match your car the 750 will work fine for ya.
Holley double pumper carbs are typically calibrated a little on the rich side "out of the box" for most street driven cars.
remember this is a racing/performance oriented carb that has to bolt on a work well on the largest number of customers cars. being a little rich is safe ( won't damage the engine and **** off the customer (you) being too lean cost money. (potential engine damage)
If you were going to sell a carb to millions of customers with lots of different car/ engine combinations, how would you setup the carb's out of the box calibration?

Basicly all you need to do is tame down the primary jetting and fine tune the power valve and accelerator pump shooters and cam selection.
What jetting is in the carb now? What intake manifold are you using?

A 650 holley is essentually a 750carbs base plate matted to a 600 holley's main body ( smaller venturii size) . So if you just got to GOT TO have a smaller cfm carb you could swap the main body for one from a 600 or a 650 to get a 650cfm carb.

But don't assume a 750 won't work for ya till you've tried it. Remember a stock QJET is a 795cfm carb.

If, after tuning it to suite your car and driving style , you still want a smaller cfm, I have a holley 700cfm DP main body. It is a easy swap (gaskets) it will make the 750 into a 700. It has the same secondary venturi diameter and a slightly smaller primary venturii which will tend to sharpen up the throttle response (Uses a slightly smaller main jet too.) if that is an issue. I'd be interested in a swap. ( main body)
But first try dialing in what you have.

 

Thank you for your reply.

Like i had mentioned it was given to me and i really would like to put it on my car, i'm sick of the carb i have on there (Quadra jet with good old Vapour lock). I used the term turn down to a 600 because that term was thrown to me, anyways i was just wondering if there is anyway i could use this carb on my car (85 Trans Am LG4, manual 5speed, Edelbrock Performer intake and cam, the rest is stock.

I'm not sure how you can tell if a carb is to lean or rich.

Also i was told that too much fuel from an oversized carb can damage the engine.

If its a simple procedure to do so, some step by step guidance would be greatly appreciated, if its not i will bring it into a carb shop and have them do it.

Thanks in advance

 

I don't know what the person you were talking to was saying, but what needs to be done is to tune the carb to run as well as possible on your engine.

1st you need to know that you aren't going to be able to just unbolt your current carb from the engine and bolt on this Holley. You will have to do some little things to attach various things like vacuum lines, fuel line, and the accelerator cable, because the Holley isn't intended to just drop in where your original carb was.

Next, your distributor won't work properly with the Holley. You'll need one that uses mechanical and vacuum advance in place of the factory one that does that stuff via the computer.

Finally, if you've got a vapor lock problem, the Holley will not fix that and will probably be less happy in those conditions than the q-jet is. Vapor lock is a fuel delivery problem, not a carb problem, so you're just going to spend time and money on something that won't address your primary issue.

Now, the only reason I am spending any time addressing this is that you've got a manual transmission, and DP-type carbs work better on manual cars than demand-type such as a q-jet, Performer, or vacuum secondary Holley/Demon. Having said that, the "issues" with putting a relatively large carb like that on a relatively small engine like a 305 are low engine speed related. Throttle response won't be as crisp as it would be with a smaller carb, gas mileage probably won't be quite as good, and the RPMs at which you go WOT will have to be higher. If you can live with those things, and don't have to worry (yet) about the negative effect on emissions legality that changing the carb will have, then go ahead and go for it.

Next you will have to learn how to tell if the carb is running rich or lean. As in "tune the carb" in the first paragraph.

 

Did I mention your stock air cleaner won't fit the Holley?

 

Why not just fix the vapour lock problem?
Like 57 said switching carbs won;t solve a vapour lock
problem. A qjet is well suited to your car and daily driving.
I would keep it and fix the prolblem especially with the high cost of gas lately.

If your Qjet is a computer controled carb, in order the run a Holley or any non cc carb you'd have to swap in a different distributor with a vac advance and mechanical advance system. pre 85 305 carbed f body are a good source.

Tuning in your holley 750 and making all the mods nessessary to switch to it will be more work then fixing the vapourlock problem on your present setup.

But to dial in your holley essentually you need to lean down the primary jetting jet size by jet size till the car stumbles while cruising down the road at a steady speed. At that point the carb jetting is "too lean". Simply go back up in jet size a few steps till the driveability comes back and you'll be just about right in primary jetting for good cruising efficientcy. But first you eed to look at what jets are in the carb now so you know where to start.
Once you tell me what jetting is in the carb now I can recomend a starting point (jetting) to start tuning.
Need to know the "List number" on the carb also. (model number) eg "4779-2"

The secondary jetting is another issue. You can worry about that later. First you want to tune the primary side.
A good starting point for a holley 750dp is #70 primary jetting and #80secondary with a "8.5" power valve.

The car will run fine with that jetting. But you can gain mileage by further leaning out the primary jetting down to about #65 jets. If at 65 jetting the car stumbles and surges (too lean), just go up a few sizes till it drives smooth again.
The car must be fully warmed up to operating temp before you test out the driveability.
You don;t need to own all 99 different holley carb jets either. The "stock jetting for a typical 750DP is 70P 80S.
So you only need a few jets leaner and a few jets richer on either side of that to fine tune it in,
66,67,68,72 jets. 78 to 85 for the secondary side,

The primary side is more critical than the seconday side.

You need to remove the fuel bowls to check the jetting #'s before ya start.
You really should just stick with the Qjet and fix the problem.

 

You know, this is why i come to this site, its full of knowledgable folks like you guys. I'm going to stick with the q-jet, haven't had any real problems with other then vapour lock. I was told a while back that vapour lock happens when the fuel inside the carb is very hot, common problem with the q-jet, so i thought maybe changing the carb, especially after being given the holley, might change things, guess i was wrong, anyways thanks a million guys for you knowledge and suggestions.

 

Vapour lock is mostly a problem of the fuel overheating in the fuel line and in the engine mounted mechanical fuel pump.
Some potential trouble spots are any rubber hoses on the feed side of the mechanical fuel pump (can collapse from execessive heat blocking smooth fuel flow from the tank to to the fuel pump. Any restriction at that point will further
cause the fuel to boil in the line ( vapour lock (pressure drop)) Another thing to check is the fuel line return to the tank. And the tank vent line and charcoal canister and canister purge control.
If the tank is not properly vented the fuel will not flow to the carb. (Rusty line) pinched line etc.

When fuel boils in the carb it is called perculation. Some causes are a blocked manifold heat raiser valve causing the carb and intake plenum to see too much heat from the heat riser. Execissive hot under hood temps during the summer
( heat soak) contribute to this as well. A blocked exhaust system will not help either ( cataylitic converter)
I'd start checking any rubber fuel line for softness an or collapseing.

The heat riser passages are designed to work with a stock cast iron intake. A edelbrock performer intyake tends to get too much plenum heat unless the heat passages are blocked off or restricted to limit the amount of exhaust gas the crosses under the intake plenum to heat the carb.
When I had a Edelbrock performer manifold I ended up blocking it off completely.

Want to sell the holley 750DP?