Holley stumble trouble

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SSG_Karg

SSG_Karg

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I’m having a helluva time trying to get rid of a stumble in my tune up. It happens at very light throttle while cruising when I try to roll into it just a little. It’s like a little hiccup then it smooths right out.
Setup:
318, comp xe268, action plus, headers. Carb is OLD Holley 4777 650DP.
71/82 jets
31/31 shooters
Blue pump cams in #2 position
Floats are at the bottom of the sight hole
Timing is 22/34 Vacuum pod is plumbed to spark port.
IAB at 1.5 out
10.5 PV
10” hg @ idle , around 20” at cruise.
Transfer slots are .045” exposed
Idles at 800
Where should I start? It runs good everywhere else and I don’t want to totally destroy the tune while chasing this damn stumble. I unhooked the vacuum pod and plugged it and the stumble was totally gone, but I would like to keep the vacuum advance.
Should I start by drilling out the IFRs?
 
sounds like a lean stumble. How much slack is in the fuel pump arm?
 
I would start by making SURE the accelerator pump is adjusted correctly. The correct adjustment will have .010" between the pump arm and the throttle arm at full throttle. From there, I would reposition the accelerator pump cam. If no luck, then proceed to different cams.
 
318willrun said:
sounds like a lean stumble. How much slack is in the fuel pump arm?
Click to expand...
I assume you mean the accelerator pump arm and not the actual fuel pump?
Zero slack in the pump arm. Shooters come in at the slightest tickle of the throttle.
 
SSG_Karg said:
I assume you mean the accelerator pump arm and not the actual fuel pump?
Zero slack in the pump arm. Shooters come in at the slightest tickle of the throttle.
Click to expand...

See post #3. LOTS of youtube videos, too.
 
RustyRatRod said:
I would start by making SURE the accelerator pump is adjusted correctly. The correct adjustment will have .010" between the pump arm and the throttle arm at full throttle. From there, I would reposition the accelerator pump cam. If no luck, then proceed to different cams.
Click to expand...
Accelerator pump arms are dead nuts. I did try the pink cams but made no difference.
 
RustyRatRod said:
Oh I'm sorry. I overlooked that somehow. You might try a bigger pump shot. Normally, a bog is too much air.
Click to expand...
I was tempted to try a bigger shooter but the next biggest I have on hand is a 37. Quite a jump from the 31 that’s currently in it. Besides, isn’t that just camouflaging the root of the problem? Not sure. Seems to me that the transition is lean and that’s what leads me to think I need to open up the idle feed restriction.
 
SSG_Karg said:
I was tempted to try a bigger shooter but the next biggest I have on hand is a 37. Quite a jump from the 31 that’s currently in it. Besides, isn’t that just camouflaging the root of the problem? Not sure. Seems to me that the transition is lean and that’s what leads me to think I need to open up the idle feed restriction.
Click to expand...

Not necessarily. Not if it needs it.....and it just may. If the bigger pump shot fixes the problem, what difference does it make? lol
 
SSG_Karg said:
It happens at very light throttle while cruising when I try to roll into it
Click to expand...
Most likely cause is
SSG_Karg said:
Transfer slots are .045” exposed
Click to expand...
Holley's Mike Urich gave the typical working range as .020 to .040"
Secondaries of course usually not exposed at all.
SSG_Karg said:
IAB at 1.5 out
Click to expand...
You mean the mix screws. Yes?
That's a good start. But the important thing is that the idle changes when you adjust them.
Reduce the throttle position, and readjust the mixture for strongest rpm in gear. Or go leaner until the rpm just begins to drop, then turn 1/4 back richer.
SSG_Karg said:
Should I start by drilling out the IFRs?
Click to expand...
NO
SSG_Karg said:
Where should I start? It runs good everywhere else and I don’t want to totally destroy the tune while chasing this damn stumble. I unhooked the vacuum pod and plugged it and the stumble was totally gone, but I would like to keep the vacuum advance.
Click to expand...
Is that 22* with or without the vacuum advance hose plugged?
I ask because at .045" t-slots that could be bringing on the vac advance a little. Probably not, but always good to be sure.
SSG_Karg said:
Timing is 22/34 Vacuum pod is plumbed to spark port.
Click to expand...
Well here's the other possibility.
18 to 22 at 800 rpm is a reasonable range for initial with your cam IMO.
34 as the mechanical maximum should be OK.
However the curve could be too quick, or the vacuum advance could be contributing at too low of vacuum.
Measure the timing at 1200, 1800, 2400 rpm, (or more if you want).
 
You can test it by backing out the mixture screws a tad and driving it to see if that helps things. If it does, it needs more fuel somewhere.
 
Mattax said:
But the important thing is that the idle changes when you adjust them.
Reduce the throttle position, and readjust the mixture for strongest rpm in gear. Or go leaner until the rpm just begins to drop, then turn 1/4 back richer.
Click to expand...
Yes, the screws do affect the idle. I set them using a vacuum gauge to achieve highest reading in neutral (4 speed).
Mattax said:
Is that 22* with or without the vacuum advance hose plugged?
Click to expand...
22* idle timing is with hose disconnected and plugged.
I will map timing curve and report back.
 
Blue cam is already an aggressive pump shot. Hate to be washing down the cylinders chasing more pump shot.
Small and gentle throttle inputs shouldn't need much shot if any.

Is tis light throttle hiccup cruising at 25, 45, or 65 MPH?
 
mopowers said:
You can test it by backing out the mixture screws a tad and driving it to see if that helps things. If it does, it needs more fuel somewhere.
Click to expand...
I’m sure it’s a lean stumble and needs some fuel added to the transition, just not sure on best way to go about adding it.
 
SSG_Karg said:
Yes, the screws do affect the idle. I set them using a vacuum gauge to achieve highest reading in neutral (4 speed).
Click to expand...
That's OK method. Maybe give them an 1/8 turn richer which should produce the same reading. It will be stronger when under load - which with the clutch is not as noticible as with the automagics but its still real.
 
Mattax said:
Is tis light throttle hiccup cruising at 25, 45, or 65 MPH?
Click to expand...
It’s more throttle position dependent than speed dependent. Mostly cruising between 2k-3k while just barely on the throttle. Above 3k is about 60mph and its not there.
Mattax said:
Blue cam is already an aggressive pump shot. Hate to be washing down the cylinders chasing more pump shot.
Click to expand...
Agreed
 
SSG_Karg said:
I’m sure it’s a lean stumble and needs some fuel added to the transition, just not sure on best way to go about adding it.
Click to expand...
Your reasoning is sound. Experminenting will prove whether its the actual issue.
The transfer slot open to .045 at idle means the slot is contributing a large portion of the fuel at idle closed;
And then runs short of fuel as the throttle opens past.

See if this works.
Holley

IMO, experiment with the smaller t-slot first.
Then depending on how that's going, measure the primary idle air bleeds. Then stick some .020 to .030 wires in them. Loop the wires down under the air cleaner gasket. That's a quick an effective way to make the idle -especially the transition - richer.
You can do the math if you want to figure the new equivelent IAB diameter with the wires. Calculate the area of the current IABs. The area of the wire. Then subtract and figure the diameter.
 
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