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Help me fix lean on power issue - Holley 4150

wsutard

wsutard

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I’m looking for some advice on tuning my Holley Double Pumper 850 on a fresh 440.

My issue is with the primary transition from cruise to power. As I give minimal to heavy throttle the engine is leaning out to the point where I get lean surge or feel the power of the engine reduce.

My AF gauge will go from reading 13 to quickly 17 or even —-. Then it will bounce around up there and eventually drop back down to 13-14.

This is all in the primary bowl. As soon as the secondaries kick in I’m immediately back down to 12.

In its stock form the Holley was very rich but never showed lean out issues. The stock jets are 80 with a 6.5 PV and 31 accelerator pump nozzle. My AF would always be 10-11 with this setup.

I’m at my wits end trying to figure out how to get more fuel from the primary’s without going rich on cruise. Outside of asking here my next step is to try a metering block with adjustable PV ports.

My current setup is as follows:
72 jets
40 nozzle
8.5 PV
Black cam on setting 1
Stock accelerator pump spring and bowl
10 vacuum
12 advance with 32-33 all in
Forged internals 440
Mew MSD box and new MSD distributor with no vac advance.
Engine dynoed at 520-550 HP.

I have tried jets all the way from 79 down to 69. I have PVs from 8.5 to 5.5. I have 31, 35, and 40 accelerator nozzles. I can not find a combo that gives me good cruise mixture but has the fuel I need for the transition into power.

I’m sure I missed something to please let me know if there is some critical info you need.
 
Before making further changes I would recommend driving with a vacuum gauge hooked up; you'll see how much vacuum is present right at the lean spot. If it's still higher than your PV when the lean spot hits, try a bigger one. If vacuum is lower than the PV, it's already open, and enlarging the PV restrictions may be the ticket. I would drill larger/ or drill-tap-use blank set screws drilled to the size you need, before shelling out for a new block. It may throw all your settings off and you'll be starting over.
**I'm going through the same exact issue with a QF750. It already has removable PVCRs so I'm gonna drill them larger; I have some blanks in case I go past the sweet spot. When I get a chance to get back into it I'll post and let ya know if it cures it...
 
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beanhead said:
Before making further changes I would recommend driving with a vacuum gauge hooked up; you'll see how much vacuum is present right at the lean spot. If it's still higher than your PV when the lean spot hits, try a bigger one. If vacuum is lower than the PV, it's already open, and enlarging the PV restrictions may be the ticket. I would drill larger/ or drill-tap-use blank set screws drilled to the size you need, before shelling out for a new block. It may throw all your settings off and you'll be starting over.
**I'm going through the same exact issue with a QF750. It already has removable PVCRs so I'm gonna drill them larger; I have some blanks in case I go past the sweet spot. When I get a chance to get back into it I'll post and let ya know if it cures it...
Click to expand...
Good idea with the vac gauge. I did do this but I was more paying attention to the off idle vac. I’ll try again and see if PV is actually in play with the lean.
 
:thumbsup: Helpful when dialing in a vacuum advance as well.
 
beanhead said:
Before making further changes I would recommend driving with a vacuum gauge hooked up; you'll see how much vacuum is present right at the lean spot. If it's still higher than your PV when the lean spot hits, try a bigger one. If vacuum is lower than the PV, it's already open, and enlarging the PV restrictions may be the ticket. I would drill larger/ or drill-tap-use blank set screws drilled to the size you need, before shelling out for a new block. It may throw all your settings off and you'll be starting over.
**I'm going through the same exact issue with a QF750. It already has removable PVCRs so I'm gonna drill them larger; I have some blanks in case I go past the sweet spot. When I get a chance to get back into it I'll post and let ya know if it cures it...
Click to expand...
Grabbed a buddy and went for a drive. The lean situation is happening between 7-11 vacuum with a majority of the leans from 9-10. My PV is 8.5. Tested on flat and also That cruising about 30 going uphill, providing some accelerator.

When I bury the throttle I’ll get a super lean second with a huge surge when the secondary’s kick in. Vac under this condition drops to almost 0 for a split second then picks back up.

Im not sure if the PV is gradual with all the way open by 8.5 or if it’s closed until 8.5.
 
Anyone can correct me if wrong, but, the PV of 8.5 is way overkill. With an 8.5, your transition would be at 17 inches of mercury. At 7-11 inches of mercury, you should be using a 3.5 to 5.0 power valve for your transition point. Put in a 4.0 and that should get you where you want to be. Number on the valve should be 50% of your transition vacuum.
 
artr8 said:
Anyone can correct me if wrong, but, the PV of 8.5 is way overkill. With an 8.5, your transition would be at 17 inches of mercury. At 7-11 inches of mercury, you should be using a 3.5 to 5.0 power valve for your transition point. Put in a 4.0 and that should get you where you want to be. Number on the valve should be 50% of your transition vacuum.
Click to expand...
I'm sorry but that's incorrect. An 8.5 PV opens only when the vacuum drops to that point and below. A 4.0 would not enrich the circuit until the vacuum dropped to 4..which would be doing nothing for the problem.

--"With an 8.5, your transition would be at 17 inches of mercury"
Not sure what you mean by this, PV rating does not determine or affect vacuum level

@wsutard: try a 10 in it, it's a quick and easy check. Get the extra fuel in a little sooner, when the lean spike hits. Don't worry about what your idle vacuum is; even if it's open at idle power valve enrichment does not affect the idle circuit. (Unless it's blown)
This is of course assuming the carb settings are correct to this point..
 
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Try jacking up your primary fuel level 1/8" and see if it makes a difference...
 
Well, I hope you find the answer to your lean problem. Don't feel the need to argue your best fix.
 
Reporting back on the 10.5 PV. Doesn’t really solve the issue. Makes me fater under cruise. I go lean and stay lean on acceleration. If I accelerate then hold then it will fall from 16-17 down to 12ish. But as long as I am accelerating I am lean.
 
Sorry for the vertical video. Hard to drive, vid, and watch gauges.

Here is a vid of my AF gauge while cruising/accelerating.

 
Reporting back on the fuel level increase.

I raised the fuel level about 1/8”. This has made the most positive change so far. It still leans out but not as much, I think. I’m going to go on a longer test drive tomorrow.

Will report back.
 
As Dave said try raising your float level. If it helps, go 'till it cures it. You may need to re-adjust if it richens up somewhere you don't want it to..
 
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Oh hell I was typing when you posted!:)
Hopefully that's your fix
 
It’s jetted like a 750. need to go back up on the jets and add restriction to the transfer slots to lean the cruise. Correct PV and PVC restriction size will also help.
Depending on how far you want to dig into this thing it can be very dialed. Your gonna need some 10-32 set screws and a 10-32 tap along with a numbered drill set. If it’s non adjustable and you really wanna tune it add 6-32 and 8-32 set screws to the list along with respective taps. Holley carbs are Infinitely tunable if you have the tools and knowledge to manipulate them. Does it have adjustable bleeds or is everything set?
 
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Reporting back on the float level change.

Bumped it up to the level in the picture. It is helping with the lean out on acceleration but I can tell that all it is really doing is richening up everything. So now my cruise is rich again. I’ll be cruising at 11af and when I give it accelerator I can see the gauge start to rise up to about 14 and then it falls back to 11 when I stop applying pedal.

I’m not certain that raising the float is actually what I want as it seems like the float level is just masking the problem. I feel like I should be able to set my jets from proper cruising af and also have enough fuel when I need it for acceleration.

Another issue that is telling me that I am probably just masking the problem is that when I floor it or provide quick large pedal my af still drops to —- I lose power and then it violently comes back.

21292BF8-3BF7-44E3-8B0F-A4A7C7C9C0BB.jpeg
 
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68 HEMI GTS said:
It’s jetted like a 750. need to go back up on the jets and add restriction to the transfer slots to lean the cruise. Correct PV and PVC restriction size will also help.
Depending on how far you want to dig into this thing it can be very dialed. Your gonna need some 10-32 set screws and a 10-32 tap along with a numbered drill set. If it’s non adjustable and you really wanna tune it add 6-32 and 8-32 set screws to the list along with respective taps. Holley carbs are Infinitely tunable if you have the tools and knowledge to manipulate them. Does it have adjustable bleeds or is everything set?
Click to expand...
It doesn’t have adjustable air bleeds nor adjustable PVCRs. I am prepared to actually drill out the holes a tad to get more flow in there. Never thought about tapping them.

I just watched a video on the transfer slot. Looks like the only adjustment there is for that is my curb idle screw. From what I can tell, If I lower that screw down so as to reduce the transfer slot opening then I might have to drill holes in the butterflies to allow more air in and transfer some of the idle work back onto the idle mixture screws?
 
That’s where the 10-32 set screws come into play. You add a restriction to the transfer slot feed and it will lean your cruise. You need more initial timing also than 12 degrees.

2CE7856A-69B8-45A6-9074-2172FC4C3D81.jpeg
9494B193-CF82-4F8A-9AEB-249D31DB55FC.jpeg
 
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