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.