OK...this is actually on a Ford 302 now...but I've had this carb for 42 years now...and it's been on so many engines I can't count between my stuff and my buddies cars. From 302's, 327's, 350's, 383's, 396's, 427's, 440's and 454's. heck yesterday I used it to fire up a 540" engine I built for a friends C2.

I got it used back about 1980 from a friends race car. Running the dates on it...turns out it was made in 1972! And it still has tight throttle shafts etc. I recently bought back the 1982 Mustang GT I had bought new in '82. I thought it was long gone and then a chance encounter revealed it was sitting in a local garage for the last 18+ years and the owner had passed away. His widow was willing to sell it back to me..so I've been working on it the last few months. Back in '82, I had removed the 2BBL that came on the "5.0 HO" engine and installed this 750 Double Pumper with an old Torker single plane intake. Ran well...but when I sold the car to my brother in law a few years later, I kept the carb and we used it on many engines. Anyway I got my hands back on the carb, rebuilt it and reinstalled it "home" on the little 302. As it turns out when the car came back to me, the engine had been rebuilt with an Edelbrock Performer package with aluminum heads, intake, 600 Edelbrock carb and their mild cam and headers. Ran OK....but I really wanted to put this back on for nostalgia's sake.

This brings me to the point. This is a relatively mild engine that holds 15'+ of vacuum. Definitely doesn't "need" a 750 DP.....but it's about the most universal carb out there and can be made to run well on anything. It came new with #70 jets in the primaries and a 6.5" power valve. When I rebuilt it I dropped it to #69 jets to see how it acted with the 6.5" PV.

It ran well....but seemed a little rich when I eyeballed the plugs, but it also had a slight hesitation when under light acceleration.

So I dropped the main jets to #67's and swapped to a 10.5" power valve.

With this engine holding 15"+ of vacuum, the PV wasn't fully open until vacuum dropped all the way to 6.5"...which is what kept it from adding fuel early enough under light throttle. A "big" cam holds a lot less vacuum...like maybe 8" or less..so the 6.5" PV will add fuel with just a slight crack of the throttle. But with the milder cam you have to keep opening throttle to have vacuum drop all that way and you get a flat spot.

After a fun test drive I found it MUCH improved! No flat spots and wails when you stuff your foot in it. Doesn't hurt that i stuck 4.10's in the rear recently also!

To add a little more....PV's are available in many settings...from like 2.5" on up to in the teen's I think. Then there are "high flow" versions identified with the large window holes vs just smaller drilled ones. And then there are "2 stage" ones typically used on emissions carbs that allow just enough fuel in steps to help them get CARB certifications for CA.

Most race engines idle at maybe 4"-6" vacuum....and if the PV is too high rated they'll be open early. This causes them to go real fat in the midrange. So in those engines you'd use a 2.5" or maybe a 4" PV. Gotta be careful though...if you have a highly restrictive setup and it pulls a lot of vacuum at WOT...they might close early and lean the mixture when you need the additional fuel.

Holley rates 4 BBL's at 1.5" of vacuum....but I've seen some engines in race classes with carb size restrictions that actually have 7"+ of vacuum at WOT. Sneaky thing is that carb flows a lot more than its rated CFM at 7" vs 1.5" but air speed is insane and it's real tough to get fuel flow correct. Anyway, back to PV's. You have the idle circuit that controls fuel on up to maybe 2000 RPM for the most part. It still contributes after that but not as much. From there you get into the main jets that flow fuel through the boosters. That will show up under cruise and WOT.

My plugs were a little dark after a highway trip...clean...but I knew I could back the main jets off a little. When tuning PV's for the street...I pay attention to how it acts on a nice cruise down the road at maybe 50-60 mph. I look for a hill and see how it acts as it starts to climb without moving the throttle any. If correct...the vacuum will drop as it starts to climb and the PV will begin to open seamlessly and add fuel. When correct you'll feel it "hold" the power" and go up the hill just fine. If you have to keep opening throttle and then all of a sudden it starts to accelerate...that's telling you it would like the PV to open earlier...hence going "up" to maybe an 8.5" or a 10.5" version. Holley uses a 6.5" as a baseline in most carbs because most of the time they go on a motor with a hot street cam that holds 10" or so of vacuum..so it's a good starting point. When you're at WOT....the PV and main jets are all adding fuel.

As far as idle goes....this carb could probably use a slightly larger idle air bleed. It holds a lot of vacuum so it "pulls" hard on the circuits. The idle screws are only out about 1 turn. They still control it OK...but typically you'd see them out closer to 2 turns. When I put it on that 540...I had to back them out 3 turns to get enough fuel to feed that beast!

Holley's can be tweaked to do pretty much anything you need them to do...just gotta decide what you're starting with and where it needs to be. In this case...it's a "universal" race carb meant for a more radical engine...that we're just retuning to do what we need for a 302" mild Mustang.

JIM

 

My 750DP (R4779), had it from new, 1972, yours looks identical. The only thing I can add to your tuning story is that I needed to put in "Idle Feed Restrictors".

Changing jets did not lean it out enough over normal driving operating range. I've installed restrictors, without looking up my notes, around .017". This makes a huge difference in cruising mixture, leans it out. (Idle feed circuit also feeds the transfer slots). Been running this for about 40 years.

I've done this to other 750 carbs and had the same good results.

If you go too lean (too large a restriction) you'll get weak throttle response from 1K to 2K rpm, and you spark plugs will be quite white.

What is an Idle Feed Restrictor? Just a piece of non-corrosive wire, about 3/4" long. I've used legs from transistors, strands of wire from "stranded wire". Place wire in the hole.

See # 25

 

Yep...used the same approach years ago on a set of 660 center squirters I made into some very streetable carbs on a tunnel ram 396. They idled clean, never fouled a plug...and had instant response when done. I've also used lead shot and just re-drilled it to what I needed...but the wires work fine.

JIM

 

If you use the wires in the IFR holes rather than playing with the low speed air bleeds, a perfect size that everyone has available is a simple staple out of an office stapler - that wire is about .016" diameter. Straighten it out, but leave the one "L" bend in it to retain it and sandwich it under the metering block gasket to hold it in place.

Lars

 

Now you tell me! LOL.

I've pulled strands of wire out of various gauges to find "just the right one". And different mfg's of wire use different size strands...

I suppose a mini stapler has an even smaller one.

JIM

 

I have avoided using staples due to the long term fear of rust. With the introduction of ethanol in fuels, this could be accelerated.
I've seen old stapled documents kept in a filing cabinet with severe rust on the staples. Perhaps where I live contributes to this.

 

Geezuz, Barry... you live in a place surrounded by salt water, where 300 days out of the year it's whipped into an airborne salt spray by a 35-knot wind, coating everything within 20 miles of the coast with saltwater. Since there is no place in Nova Scotia further than 20 miles from the coast, it means that everything rusts there, including plastic, glass, solid gold jewelry and human skin...

Lars