Holley transfer slot adjustment on secondaries

[gsxrken]

I've done some digging and have come up with the usual internet confusion.

I have a 750 double pumper (part #4779-9) although it now has a 750 HP main body on it (part # 134 300C). I have my carb off and am chasing a tip-in stumble when I get back on the gas after a gear change. I know, I know... accelerator pump circuit. But I did notice my primary transfer slot was exposed the proper .040, but that my secondary slots were not exposed at all. Should I also tip the blades in a bit so I have a .020 - .040 exposed on the slot, or leave it alone?
Asking because a.) I have it off, and b.) the adjustment screw is only accessible with the carb upside down. I'd like to try to nail it to spec now, and tweak with the idle screw.

Any ideas?

 

[camachinist]

Your carb has four corner idle, correct?

Generally, I've always adjusted the secondary blade to balance idle fuel/air delivery and mitigate idle issues, rather than addressing stumble problems.

Some stuff to check:
Proper clearance at WOT for the pump arm to lever. The service manual spells out the clearance.
Proper operation of the nozzle check valve/ball
Adjust primary pump cam and/or nozzle size
Experiment with larger pump capacity

Since your issue is between gears, assuming you release and re-apply the throttle, your focus should be on the primary side of the carb. You can duplicate rapid release and application off the engine and observe operations.

Other than properly balancing idle fuel/air between primary and secondary (assuming secondary idle circuit), my focus, assuming the carb is adjusted correctly to factory specs, would be the primary accelerator pump circuit.

When you perform high rpm shifts when performance driving, I presume operations are normal and this appears to be a street driveability issue, correct?

 

[gsxrken]

Thanks Pat. Good points about the primary circuit during part-throttle let offs for street shifting. I do expect the slight hiccup to be accel shooter size related. I'm finely adjusted and jetted well, this is just a driveability thing that only I can notice. I picked up a 35 shooter I'd like to try- easy enough.

I spoke with Holley tech today and he confirmed that the secondary transfer slot should also be exposed in the .020-.040 range. I'm going to set it to .020 and then dial my probably now-too-high idle back with the primary idle screw.

 

[camachinist]

Good info. I hope that secondary idle screw turns easier for you than it did on any of the old Holley's I've worked on.

Let me know how much screw it takes to get it into that spec. I'm curious.

If that doesn't fix, get back ahd we can work it further. Sometimes such stumbles are pullover issues in the pump circuit, where the fuel gets pulled over during normal ops and there's a momentary lack of fuel before everything fills up again. I've having that kind of issue on my 4100 right now, only during light off-idle throttle application. Too d#mned hot to work it now LOL