What is Holly 750 Carb Fuel Pressure suppose to be?

[Blown331LX]

My 1990 GT is topped with a Edlebrock Vic Jr, and a Holly 750 dp. I have the Holly Red pump which is self regulated at 7 psi, and usually pushes 7-9. The carb has the power valve blocked off and Im not sure what size jets. But it runs pig rich at idle. Should I drop the pressure to 4-5 psi? Will this change the performance at WOT and make it lean? Im not a carb specialist so I do need some help, all the carb tuning was done by a Mustang Guru friend of mine and it runs great, but just rich at idle.

 

[Shamus O'toole]

I run 6 1/2 psi through a QF 750P. If you car was tuned more for WOT then drivability and (lol) fuel mileage, its gonna be rich at an idle. Such is the nature of the beast.

 

[Blown331LX]

I bought a Holly regulator yesterday, hoping lowering it some will help. If that pump was pushin more than the supposed 7 psi, its regulated for than maybe thats the problem??? I had an older Holly Red pump, and the plunger stuck and it was pushin 14 psi and smoking, so I replaced it with another brand new one thats now sending 7-8 and it is still running pig rich at idle.

 

[rd]

The fuel pressure should not be a big factor in the idle mixture.

If you have decent airflow through the carb at idle, working with the idle mixture and idle air bleed should be all you need.

I would not lower fuel pressure below 6 or so, since you need it at WOT.

I would suggest going back to a power valve.

 

[Shamus O'toole]

+1 about the power valve. If you have decent vacuum at an idle, then it's not nessesary to block of the power valve.

 

[PUNISHER RACING]

I would not recommend not running a power valve in the primary side get yourself a vacuum reading on your car at warm idle in gear and set your power valve up to be half of that. If you are not running one in the rear up your jet size about 4 sizes to start. Set your fuel pressure at 6 psi and start wit a jet sizing of 70/75 to start.

 

[Blown331LX]

Is the 750 Dp the proper carb for my combo? I have a stock 1994 GT 302 bottom end, F303 cam, Patriot Heads ( same as Performer RPM), a Vic Jr intake and BBK 1-5/8 long tubes.

 

[PUNISHER RACING]

it will work when set up properly

 

[Blown331LX]

Hey man thanks, it the same thing I thought. Im assuming the jets and power valve will likely be comperable to a 650, with the added air flow of the 750. Im gonna give my Local Mustang god another crack at it, and then I might pull out the old school books and have a crack at it myself. I knew Holly really well, and trained by Nascar guys but I havent had one apart in almost 4 years, and really hate myself for it.

 

[jake2582]

1) i always take my power valves out and throw them through the woods

2) if it is running rich at idle why dont you lean the idle mixture with the "idle mixture screws"? ya know im no expert but thats where i would start

 

[Blown331LX]

The idle air screw is the top horizontal one in the side of the metering block correct?

 

[jake2582]

yes. should be two on the front for sure. and two on the back if it has 4 corner idle.

i would run them all the way in and see what they are.
i usually run them both in and back them out 13/4 to 21/2 turns if it only has them on the front. (in is leaner and out is richer)

 

[PUNISHER RACING]

1st key your carb is too big run your idle mixture screws in the whole way and see if it still idles perfect if it does too big. Seriously though take a vac. reading throw a pv in the front metering plate 1/2 the reading, block the rear and up your jet size 4 sizes to start. You will be alot happier with a street car with a pv.