Need help with lean stumble on Holley DP

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Walker434

Walker434

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My 71 Dart has a 5.9 magnum with air gap intake and Holley 750 double pumper. Has always run great but I've been fine tuning lately with an air/fuel gauge. Been really nailing down my tune trying to get 14.7ish at cruise and still keep 12.5ish at WOT.

Friday night I drilled out my power valve restrictor channels from 0.59 to 0.625. Had tApe around my drill bit to make sure I didn't drill too deep. Put it back together and it didnt seem to idle correctly like it did before. Good knews is that I now have the cruise AFRs I'm looking for and WOT AFRs I need. Bad new is the idle doesn't seem quite right and it now has a lean stumble at very light throttle. Will go into the 18s for a second until you give it more gas then it's normal.

Maybe has nothing to do with drilling out PVCRs and just picked up trash in the idle circuit? On light acceleration the carb is still using the idle circuit right? Maybe trash in the needle and seat? I'm learning a lot about tuning carbs but I just wanted to throw this out there to get you guys thoughts. If I can get this lean stumble figured out this carb will be dialed in! Let me know your thoughts. Thanks in advance.
 
I'm having the same issue. Do you have a vacuum gauge? On mine when I try to lean out the primary jets to get the cruise AFR correct, I get the same lean stumble. My problem is the car idles with 8-8.5 inches of vacuum, and I have a 6.5 power valve. At light throttle the vacuum only drops to about 10, and the power valve never opens, hence the lean stumble. If I use more throttle the vacuum drops below 6.5 and no problem.
 
Once the throttle plates crack open you are off the idle circuit. The throttle plates must be completely closed to go into the idle circuit
 
You have some debris still in there. I'll assume the IFR is in the lower position of the metering block.
 
Most street cars run on the idle circuit from idle to approximately 2200-2500 rpm. If you lean the idle circuit to much you can develope a tip in stumble. Don't tune to a number on a gauge, tune the motor to run good. AFR gauge is a tool. Try fattening the idle circuit a little and see if your stumble improves or goes away.
 
I also think it is debris in the idle circuit somewhere. Haven't had this stumble until Friday when I drilled my PVCRs. I'm guessing it's a coincidence, or possibly some of the tiny metal shavings? I used air to blow out the entire metering block after I drilled. Could a dirty needle and seat cause this? Seems like that is usually more of a flooding issue.
 
Walker434 said:
I also think it is debris in the idle circuit somewhere. Haven't had this stumble until Friday when I drilled my PVCRs. I'm guessing it's a coincidence, or possibly some of the tiny metal shavings? I used air to blow out the entire metering block after I drilled. Could a dirty needle and seat cause this? Seems like that is usually more of a flooding issue.
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Anything you did to the PVCR shouldn't have an effect on idle. It must be a chip or something lodged in a passage.

I *THINK* BLP has those tiny cup plugs used in the top of the metering block. You can pull those and blow out the emulsion wells and holes from the top.

That is ASSuming you have Holley metering blocks.
 
Walker434 said:
I also think it is debris in the idle circuit somewhere. Haven't had this stumble until Friday when I drilled my PVCRs. I'm guessing it's a coincidence, or possibly some of the tiny metal shavings? I used air to blow out the entire metering block after I drilled. Could a dirty needle and seat cause this? Seems like that is usually more of a flooding issue.
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Yes a dirty needle and seat usually results in flooding and will show up with an incorrect bowl fuel level. Your Idle Feed Restrictor is approxiamately .033-.035 so it doesn't take much debris to screw up the flow.
 
Yellow rose. I was also thinking that drilling out my PVCRs couldn't affect the idle circuit. I do have the stock Holley metering blocks and I think they have the tiny screws in the top so I will try blowing those out. Haven't had time to take it back apart yet but hopefully tonight. Thanks
 
Mderoy340, I will also clean out my IFRs when I take this apart. Hopefully an easy fix. Thanks for the tips and I'll report back with my findings.
 
Just an update. Removed the primary fuel bowl and metering block las night for cleaning. Used carb cleaner and thorowly cleaned every passage. Removed idle air screws and cleaned those as well. Put back together and my idle is back to perfect. Cruise no longer has a light throttle stumble. The only lean surge left is light throttle roll from a stop sign. Doesn't stumble but you can see it go a little lean by the AFR gauge. Maybe this will cure itself with more driving. Overall back to normal. Thanks FABO!
 
Are the IFRs in the top or bottom of the metering block? If they are in the top moving them to the bottom will help. A fuel jet is more stable below float level. Old holley blocks have the IFR in the bottom. For some reason most new billet blocks have them up top.
 
Walker434 said:
Just an update. Removed the primary fuel bowl and metering block las night for cleaning. Used carb cleaner and thorowly cleaned every passage. Removed idle air screws and cleaned those as well. Put back together and my idle is back to perfect. Cruise no longer has a light throttle stumble. The only lean surge left is light throttle roll from a stop sign. Doesn't stumble but you can see it go a little lean by the AFR gauge. Maybe this will cure itself with more driving. Overall back to normal. Thanks FABO!
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Very cool.

The slight lean my be an accelerator pump adjustment or you may need a touch larger squirter.
 
The IFRs are up top on mine. It's a newer 4779-9 model. I think I bought it new around January 2014. Not sure how to go about moving the IFRs to the bottom as you speak of. I'm still learning the carb tuning stuff so not sure what you mean on that. I think I need to buy that Holley tuning book I've heard people talk about.
 
Walker434 said:
The IFRs are up top on mine. It's a newer 4779-9 model. I think I bought it new around January 2014. Not sure how to go about moving the IFRs to the bottom as you speak of. I'm still learning the carb tuning stuff so not sure what you mean on that. I think I need to buy that Holley tuning book I've heard people talk about.
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I have an application that likes the IFR up where yours are, but it's pretty rare to find one that doesn't like them lower. I suspect mine likes them up higher because my cam is 255 @ .050 on 340 inches.

It's pretty simple to move them. It requires that you remove the plug from the top where it is and thread the hole at the bottom of the metering block to 6/32 IIRC. Then you get some brass 6/32 set screws (I get them from McMaster-Carr) and you get some number drills and you can start by matching the IFR size you already have. Then tune from there. If it's a bit rich at idle you can go down .003-.004 on the IFR and that's as simple as drilling the set screws and screwing them in.

If want to go whole hog, you can remove the press in air bleeds and IIRC tap the holes to 10/32 and make it so you can change the main air and idle air bleeds.
 
what does it do when you hammer it, does it stumble ?
 
Nope it's perfect at cruise and wide open throttle. Basically got all figured out except the extremely light throttle froma stop. Going lean then but all other areas are great.
 
Walker434 said:
Nope it's perfect at cruise and wide open throttle. Basically got all figured out except the extremely light throttle froma stop. Going lean then but all other areas are great.
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At that condition the pump discharge may be nil to dribbling down the venturi, that transition is supposed to be covered by the transfer slots or "T-Ports". If the throttles
are opened too far to maintain an acceptable idle speed, You will be setting the idle mix with the slots partially exposed already, so they can add very little at "tip-in".
If they are, You may need to add or enlarge holes drilled in the throttle plates, even a 1bbl slanty has one drilled in it. Even though I'm certain You've checked, a slightly
low fuel level will cause a sag, because it delays the start-up of the main circuit discharging. Just some things to look at...................
 
Have you adjusted your T-port sync?
It may be that your throttle blades are a touch too far closed, and you have compensated the lean transfers with the mixture screws. It may be that if you increased the idle speed 50 rpm, the stumble goes away. It may then be that you can lean the mixture screws a touch.
And if that doesn't get her, then as already said, a touch higher fuel level might do it.
 
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