Holley stumble trouble

-

SSG_Karg

Enhanced member
Joined
Feb 24, 2010
Messages
1,737
Reaction score
1,936
Location
Pennsylvania
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.
 
sounds like a lean stumble. How much slack is in the fuel pump arm?
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.
 
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.

See post #3. LOTS of youtube videos, too.
 
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.
Accelerator pump arms are dead nuts. I did try the pink cams but made no difference.
 
Oh I'm sorry. I overlooked that somehow. You might try a bigger pump shot. Normally, a bog is too much air.
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.
 
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.

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
 
It happens at very light throttle while cruising when I try to roll into it
Most likely cause is
Transfer slots are .045” exposed
Holley's Mike Urich gave the typical working range as .020 to .040"
Secondaries of course usually not exposed at all.
IAB at 1.5 out
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.
Should I start by drilling out the IFRs?
NO
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.
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.
Timing is 22/34 Vacuum pod is plumbed to spark port.
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.
 
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.
Yes, the screws do affect the idle. I set them using a vacuum gauge to achieve highest reading in neutral (4 speed).
Is that 22* with or without the vacuum advance hose plugged?
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?
 
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.
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.
 
Yes, the screws do affect the idle. I set them using a vacuum gauge to achieve highest reading in neutral (4 speed).
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.
 
Is tis light throttle hiccup cruising at 25, 45, or 65 MPH?
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.
Blue cam is already an aggressive pump shot. Hate to be washing down the cylinders chasing more pump shot.
Agreed
 
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.
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.
 
-
Back
Top