750 DP Part throttle lean

I’ve been chasing a lean condition and not sure the best way to fix it. I’ve got a 750 DP with a Proform main body and Holley metering blocks. #74 in the primaries with a 6.5 PV and #84 with no PV in the secondaries. The motor idles at about 1100 rpm with 9-10” at 14 afr. Cruising at 55 mph at 2600 rpm the afr is about 13.5. WOT the afr is about 12.5-13. The problem area is part throttle about 1/3 to 1/2 throttle it goes really lean, about 16-17 and you can hear it miss. It happens when pulling away from a stop or accelerating to pass on the highway.
I’d love to get a Sniper EFI but that is a ways down the road for me.

Never seen that problem on a 4150, 4500 yes and that is fixed by richening the transfer slot jet and mixture screws. You could try turning the mixture screws out to richen in 1/8 turn increments til you get it balanced out between a clean transition and idle. Could be in the squirter or squiter pump cam also. Try whats easiest first.
The pump gas we have today don't help either. I sometimes have same problem in hot weather with straight pump gas. If I add some race fuel problem goes away.

Thanks! I’ll try richening up the idle first. I forgot to mention I have 30 cc pumps with #31 nozzles and pink cams set in the number 2 hole.

How did you come up with a 6.5 power valve? that's way too late of an opening. You need to measure CRUISE vacuum and set the power valve to that. Everyone tries to cover a tip in issue with accelerator pump and transfer slot. But when you open the power valve that late, you get tip in issues.

Never set power valve opening by idle vacuum. That is wrong. Set your cruise A/F ratio with the primary main jet. Then control tip in and WOT with power valve opening and power valve channel restriction. Once you have your cruise A/F ratio set, never set WOT with the primary main jet. Use the PVCR and lean or fatten up WOT with that and your secondary main jets.


I think you are a bit fat at cruise. I'd like to see a plug or two. I'd bet you can lean out the mains a bit more and the fix the tip in and WOT with power valve tuning.


Again, do not set PV opening from idle vacuum. I know Holley still teaches that, and they are still wrong.

Adding to what was posted above....... get a vacuum reading while the motor is in the 16:1 range.

Also, what list number carb did the metering blocks you’re using come from?

That could be the issue or at least part of it. In any case it is an easy thing to test. If you're cruising at 18 inches of vacuum then try a 12 inch power valve to see it helps. If you are not racing the car then put a PV in the secondary side also. Race car carbs are optimized for WOT so no surprise that you have issues at part throttle.

Cruising at 2,600RPM, it is likely that you are on the main circuit. And I agree that 13.5 is a bit rich for cruise. Smaller main jets will probably bring that mix down, but could exaggerate the lean transition problem. In order to fix the lean transition, you have to know which circuit is the culprit. It could be the slots, it could be the power valve, or it could be the emulsions or the bleeds delaying the mains. Sorry to make it complicated but you have to know what is wrong before you know what to fix, let alone how to fix it.

You may use a timing light shining into the carb and try to duplicate the lean spike to see if the mains are in at that point. As a starting point, it would be important to know if the boosters are engaged or not at the point of the lean spike. If they are, then the power valve or PVCRs would be where I would look as you are on the main circuit at that point. If not, then I would look to bringing the mains in sooner and/or fattening up the slots.

How many turns out on mixture screw at good hot idle?
At idle and cruise the carb is on the mixture screw, idle air bleed, idle feed restrictor and transfer slot till you get over 2500 rpm or so. Then the main jet should be coming in and adding fuel from there.
If your over 2 turns out on mixture screws I would look at idle air bleed and idle feed restrictor.

Some of our cars cruise on the mains and some cruise on the transitions. The 2,500RPM point is certainly not set in stone. Which is why I suggested he see if the boosters are involved at the point of his problem. He also has the added variable of different brand parts (main body vs metering blocks) and how they interact together. It might have been easier if he were starting with an unmolested factory assembled carb.

I would try fattening up the primary transition slots a tiny bit at a time until your happy, NOT a bunch to start with

another way to see if you're cruising on the mains is to do a big main jet change (reset the idle mix after) then go for an easy cruise and see what rpm the AFR difference shows up..

I guess I'm confused ands need to read the OP again. It's rich at cruise and has a tip in issue. How is that fixed with jets or Tslot? That doesn't make sense to me, but I've been wrong more than right. If it stumbles on tip in, it's either like mentioned above that the pump shot is too small or he needs fuel enrichment from the PV sooner.

Or so it seems to me. But I'm wrong. A lot. When he gets it tuned I hope he tells us what fixed it.

After rereading the original post your correct on which part of the carb to work on, it isn't in the transition circuit at 2600 RPM
OP, try the power enrichment circuit first, power valve opening vacuum rating and maybe the power valve enrichment channel size also

Find the idle fuel restriction. With fine/ precission drill bits measure the opening. Then open it 1-2 sizes. Not more that .030 This is the tip in fuel. Not the OV or the main jet, or the accel pump.

On some metering blocks it is visible from the front, others it is covered with a brass plug on top. ( this is a hard to drill version)

Heres where the IFR are located, usually in the top, but could be in the bottom as pointed out. I had same issue with my Dominator and tried several power valves, PVCR's . What fixed my issue was getting the T-slots correct or at least close. I still get the issue on hot summer days with pump gas, but if I add some race fuel to the pump gas issue goes away. My cruise was on the rich side as well (13.5-14)just as I like it and still had the lean tip in. It is better to have settings on the fat side vs the lean side. Your engine will be much happier.

"Cruising at 55 mph at 2600 rpm the afr is about 13.5. WOT the afr is about 12.5-13. The problem area is part throttle about 1/3 to 1/2 throttle it goes really lean"

It appears that the reference to 2,600RPM is for the cruise ratio. The lean issue is part throttle transition. Which suggests to me that any one of several different circuits could be the culprit, including the ones mentioned above. But the OP needs to identify the offending circuit to correct, although he may cover up the problem by fattening up another circuit as is often done with the accelerator pump.

I'll give a recent example similar to this that I dealt with. After fighting a slight, but noticeable tip-in lean spike for a while, I finally called ThumperDart. After a few questions he had me run a test or two which determined that my lean spike happened right before the point the boosters activated. He suggested that I switch the top emulsion jet with the plugged second one. What he wanted was to bring the main circuit in a bit sooner. That did it! Fattening up other circuits may have covered up that issue, but that usually causes other issues. So, it's important to know what the circuits are doing and when they are doing it.

I agree Dave, its hard to know for sure.
My sense from the description is the problem is early portion of the main.
Could be in the timing too.

My suggestions:
Note the vac, rpm, AFR at interstate cruising speed. (ie 65 mph)
Compare with same at county highway speeds (which will be less throttle) such as 35 and 50 mph.

If the metering blocks have 3 or more "emulsion holes", plug two of them. A little too much air early in circuit startup can make it go rich-lean-rich.
Keep the one near fuel level and one near the bottom open. They should be around .026-.028" dia.
If they are really small, then 3 e-holes can work. I think Dom or Mark have posted sizes that can work with three holes but don't recall if that was for 4150s or dominators.
The three hole Holley block I got from QF had 3 holes around .028" dia. I plugged the middle ones.

Note: As throttle is opened from steady cruise at highway speeds, the AFR should go leaner and the vehicle should accelerate (in other words, power output increases).
A lot of us grew up thinking more throttle makes more power by making a richer mixture but its not true. Only when near full throttle does it need to go richer.

Thanks everyone for the suggestions. Before I change anything I'll hook up my vacuum gauge next to my tachometer and a/f gauge and record a video of the issue I'm having. That might help get this sorted out. I'll also post answers to some of the question that were asked in this thread. I should have a video of it in the next couple of hours.

I did a short test but it didn't turnout as good as I would have hoped. It started to rain so I wasn't able to get a better video done. The frame rate of the camera makes reading the A/F gauge almost impossible.
The original main body is a 650 4777-7 with a 2 corner idle. The secondary metering block has no option for a power valve. I've got the idle mixture screws set at 1 1/2 turn out.
Initial timing is 22 deg and full advance is 34. I made my own bushing to give me 12 deg of advance. I have a MSD 6AL with their 8545 distributor with a heavy silver and light silver springs.
The cam is a Comp XE275HL with 231/237 @ .050 installed at 106.
I think I answered all of the questions that wouldn't be answered in the video.

Honestly, I couldn't hear any tip in issues, but my wife says I'm as deaf as a board. From what I THINK I saw, you are well over 15 inches of vacuum at that cruise RPM. Whether it's the issue or not, I think you need to open the power valve sooner, and work on cleaning up the cruise A/F ratio.

There is no substitute for data logging. Why guys try to video their way to A/F data is beyond me.

If you cruise on the mains, when you lean the cruise that means smaller main jets which means you will need to open up the PVCRs to get the WOT mix back where it needs to be. And if the power valve is too late now, it will be a bigger issue with smaller main jets. I am not suggesting that that is the wrong approach. I am just noting the additional steps that need to accompany that route.

At what RPM is the timing all in? And with that big a cam, more initial may be called for if the starter will take it.

Whats with the first frame? Are you really idling at 1300 rpm with 9 inches of vacuum?

First thing that stuck out to me was, IMO, there is no reason for the engine with that camshaft to idle at 1200+ in P/N.

That engine should easily idle at 950 and a pull down to 850-900 in gear if the converter is OK. Might be a ground up issue to some extent, too much plate opening at idle.

I’m going to revive this thread instead of creating a new one. Now that I have a bunch of time on my hands I thought I would go through the carb and make sure everything is clean and the way it should be.

In the video above it looks like the engine is idling at 1200 rpm. Sometimes the needle will stick and not drop below that number. If I tap on the gauge it will usually drop to where it should be. I didn’t really notice it when I made the video. I was more focused on the vacuum reading.

I took some pictures of the disassembled carb. Maybe you guys can help me figure out if something is not quite right. I might look at buying some replacement metering blocks and was wondering if my base plate would support 4 corner idle.

I have the throttle blades set so that the transfer slot looks “square”. Is this where I should have them?

Here you can see what looks like some flashing partially blocking the channel. Is it supposed to be like this?

My Proform carb is identical to the 650 body on the primary side but the secondary side is different.

Here you can see how far the primaries open before the secondaries. Is this about right?

Are you using 650DP metering blocks and baseplate with a Proform 750 main body?

My experience is that the 650 metering blocks don’t play very well with that body.

The primary block from a 4777 has emulsion tubes taking up a lot of space in the main well.
And as I recall the angle channel on the secondary block is really more of a “horizontal” channel way at the top.
I don’t know if they were all like that.

4777’s never had 4 corner idle so a couple of extra holes need to be added to the stock base plate.

I’ve had a proform bodied carb built from a 4777 on the dyno before...... it didn’t run well at all.
On that carb, a pair of metering blocks fixed it.

QFT makes a kit with a pair of blocks if you haven’t gotten replacements yet.

Edit- looks like your secondary block has a normal angle channel.

If your primary block has the inner two brass plugs up on the top of the block looking “flat” instead of a “cup”, those are the tops of the emulsion tubes.
If all 4 plugs are “cups”, then it doesn’t have the tubes.

I built a number of those HP750’s from scratch when those main bodies were a pretty new thing.
I would use stock 4779 metering blocks and the stock 4 corner idle baseplate.
Those carbs always ran pretty well.

I won't and don't open the throttle plates enough to expose the transition slots, you want the motor to idle on the idle circuits only, not the transition circuits, correct The myth about the transition slots needing to be exposed at idle has been posted on here way to many times
I think if you open the throttle plates enough to expose the transition slots I also think that affects the power valve opening at low manifold vacuum

How much t-slot is exposed on the secondary blades.....I have found in SOME cases that the square ft. and rear t-slot exposure can help across the board and I set em all square to start then set the mixture screws accordingly. I also like the pink cams on the #1 setting......

On Dommy's, never any t-slot exposed but on 4150's always or it goes lean till the mains/boosters kick in but that's just my way and doesn't mean it's for everyone or every combo...

Thanks for the replies. All of the brass plugs are cupped.
I think I’ll look for for a nice 750 to use. I don’t want to mess with drilling out the baseplate and getting new metering blocks.

Do yourself a favor and step up to the technology that's available today w/billet blocks and bases which are lighter and better in every way.......Maybe even a alum. main body or better yet but a new carb w/all the features anyone will ever need or possible use.......No clue why folks keep messin w/the old junk unless you're broke of course then I get it.....

If you’re going to buy a whole 750 to use with that Proform body, make sure it’s a new enough one that already has 4 corner idle.
The older 4779’s didn’t have it.
I don’t know what dash number it was when the 4 corner idle was std.

I can’t quite make out what your stock main body number is.
4777-7?

That’s what I used to make my dyno carb out of.
I’m not sure what vintage the metering blocks are, but I believe they’re earlier than a -7 though.
They have the emulsion tubes in the primary block....... but if you’re working with a matched set of components from a 4777-7, then it would appear they had done away with that by that the time the -7 was made.

I used an aftermarket base plate, and modded the rear metering block and main body to have 4 corner idle.

I built this carb out of a new Holley 134-300S main body and a bunch of other QFT parts.

Nowadays you can buy a Street Q or Brawler carb for less that it cost to put this together.

I built a HP950 from scratch about 20 years ago using a genuine Holley HP bare main body.
When you bought one of those, bare meant “bare”.

For the end user, the only reason to try stuff like this is because you want to play with it.
There are some pretty good deals out there for carbs these days.

I see you worked over the venturi's to get rid of the flashing.......

That’s one nice thing about the boosters not being there.

I’m sure that’s a non-issue with a nice billet body.

The early Proform bodies would often have a pretty noticeable step there.
And usually....... only on one side.

Agree, and yes the billets don't suffer from casting flaws but not everyone needs one although on a flow bench they are dead consistent from venturi to venturi and have no casting issues. As you mentioned the Brawlers and a few others can be tuned in nicely and perform very well after a few mods.....

Thanks for the info and pictures. I've got a lead on a nice Holley 850HP Street. Looks like I could put my 750 main body on it and have it better sized for my combo. Eventually I would like to get a Holley Sniper. I currently don't have any type spacer under the carb. I do have room for a 1" inch spacer. What would you guys suggest for something on top of my Performer RPM?

As far as “850’s” go, the street HP850(82851)is an odd carb.
It’s the only one I can think of that uses the 1.688 baseplate(instead of the 1.750).

It has the big venturi(1.563), but the small baseplate. This is the same configuration as the older “830” carbs.
I was never a big fan of that set up.

The original HP950 is the exact opposite. It has the smaller venturi(1.375) with the big baseplate(1.750).

In any case, since the Street HP850 uses the small baseplate, your Proform body will work on it.

Brawler race series sizes:
67199 650 = 1.250 venturi/1.687 baseplate
67200 750 = 1.390 venturi/1.687 baseplate
67201 850 = 1.390 venturi/1.750 baseplate
67202 950 = 1.450 venturi/1.750 baseplate

There are several configurations of various “850” and “950” carbs utilizing different venturi sizes and baseplates.
They don’t flow the same....... or behave the same.
It’s usually best to reference the physical sizes of the venturi and baseplate when talking “850’s” and “950’s”, to ensure everyone is on the same page with exactly what size carb you’re really talking about.

For example...... a few “950’s”
Holley 80496 - 1.375v x 1.750
Holley 82951 - 1.563v x 1.750
Holley 80805 - 1.600v x 1.750
Brawler 67202 - 1.450v x 1.750

Thanks! I called Holley yesterday to get the same info. They said it would work but didn't provide the amount of info that you just did. I should have asked here first!
Do you see any issues with this idea? The carb is a 82851.

Having not used a 82851 myself I can’t say for sure one way or the other.

As long as all the transfer and idle feed holes in the baseplate line up with the ones in Proform body, I don’t see any reason why it wouldn’t work.

Of course there’s no guarantee the Proform body will work any better than the 82851 body.
It might not even work as well on your combo.
But ........ you’ll have both to try.

Thanks! I'm going to take a look at it today.

Here is the result of the 850HP Street with a Proform body. I went with #72 in the primary and #84 in the secondary. The largest PV that I currently have is 6.5 so that’s what’s in there. I’ve got pink cams in the first hole with 30cc pumps and 031 nozzles. What secondary throttle plate opening should I start with? I’ve seen some people take out the secondary throttle adjustment screw and put it in from the top. Will this work? How many turns out from seated should I set the idle mixtures screws to start with?
Thanks again for the help!

I think your jet spread is to big unless your power valve fuel holes in the metering blocks are real big
I think the old adage of having 6 # bigger jet size in the rear (#72 front with #78 in the rear) with no power valve is a good place to start
I am also against opening throttle shafts enough to expose the transition slots on any Holley type carb
On the idle mixture screw initial setting I start with two full turns out from GENTLY seated and adjust from there