Holley tip in stumble

[BLstangin]

Just got a 460 running that was build years ago and has been sitting for a while. It has always had a light stumble right at tip in the accelerator pump had a pink pump cam in the primary side in screw hole 1. I'm not sure what size shooter is in it but it's a 750 double pumper and it runs fantastic every where else and is just there for a split second at the hit of the throttle then is gone, how do I tune that out? Would moving the cam up to number 2 help?

[Firedome8]

Try it and see. Make sure it discharges fuel as soon as the throttle is moved,,,.try turning mixture screws out a 1/4 turn. ..

[lefty o]

make sure the accelerator pump linkage is set up correctly, so that it gives a shot instantly when the throttle opens. way too many people leave slack in the system.

[BOOT]

Tip-in stumble could be tons of stuff in the carb to timing.

Are you using a spacer if so what type? What type of intake? More specs may get you more possible solutions.

[Tuner]

Turn the pink cam on the primary upside down with the screw in the # 2 hole in the lever and # 1 hole in the cam. You will need to select best fit to the pump lever and also adjust the pump lever, so the 'knee' instead of the radius pushes the pump lever. This roughly acts similar as the blue pump cam and will give much more pump shot just off idle. Be sure to adjust the spring so there is preload on the diaphragm, no slack when the throttle is closed and it is not bottomed out at WOT.

[statsystems]

Turn the pink cam on the primary upside down with the screw in the # 2 hole in the lever and # 1 hole in the cam. You will need to select best fit to the pump lever and also adjust the pump lever, so the 'knee' instead of the radius pushes the pump lever. This roughly acts similar as the blue pump cam and will give much more pump shot just off idle. Be sure to adjust the spring so there is preload on the diaphragm, no slack when the throttle is closed and it is not bottomed out at WOT.

If I have a pink secondary cam can I do the same and flip it over and use the #2 hole in the lever and the #1 hole in the cam? If not, what's the best way to address the secondary pump cam?


Thanks

[F-BIRD'88]

After you try all the accelerator pump tuning and tweeking if it does not fix the stumble,

Check the PCV valve. If it is wrong for that motor it can upset the carb idle and off idle circuit.
Does temp blocking the PCV valve or restricting the flow help?

The carbs idle circuit may need a slight tweek to get the air fuel ratio right at idle and just off idle. (transitiion) This often can be tweeked just by slight adjustment of the primary idle air bleeds.
Yours may be a just a hair too big creating a off idle transition lean stumble.
Hold your 2 fingers over the pri idle air bleeds to restrict the air flow a bit .
now tweek the throttle rpm... A AFR gauge+ O2 sensor can help you see the afr shift and the effect on that by tweeking the pri air bleed flow.
This can happen with some open plenum intake manifolds.
All made worst by a big cam. More idle base timing helps too. (distributor recurve)
But again something as simple as the wrong PCV valve can throw you for a curve.
The primary and the sec throttles must be adjusted correctly to give correct transition slot exposure at idle. too open and or too closed upsets the idle transition AFR creating a flat spot too. Take the carb off and look. .020"-.030" of throttle T slot exposure is about right.
You then tweek the idle rpm speed with the ignition base timing and the air flow thru the PCV valve internal orifice at idle to get more or less idle air flow and idle rpm speed.
One or more of these are out of wack.
Be sure the intake manifold is all warmed up before evaluating idle quality.
A cold non heated plenum is going to be more pesky.
A bad design air cleaner can throw it all off too.
Make sure the ignition system is good ( plugs, wire cap rotor) and there are no manifold/carb vacuum leaks. Some times the idle air bleeds get gummed up. Blow out with shop air to clear.

But I bet if it is the air bleeds they are a hair too big.
Screw in air bleeds or pressed in? Are the pri idle air bleeds Bigger than a .062" (1/16" drill bit) ??? Does tweeking the flow+/- a bit. with your 2 fingers over the bleeds, while working the throttle rpm correct the stumble?

[Geoff2]

Check the idle timing BEFORE touching the carb! Depending on how big the cam is, it might want anything from 15 to 50* at idle [ yes 50*!! ].
A good middle of the road idle timing is 25*, achievable by using vac adv connected to manifold vacuum [ MVA ] to add to init timing. Eg, if you run 12* init add about 13* with MVA [ not critical ]. If you do not have vac adv, advance init timing to about 25* to simulate what MVA would do. I would like a dollar for every off idle stumble I have fixed just by getting the idle timing correct....

[BLstangin]

Thanks guys, the idle is set at 700 rpm and it idles beautiful and if you shut it off hot it will start with the bump of the key. It has a very small cam in it and a wiend stealth intake on it. The idle timing right now is 14 degrees and it has a mechanical msd dist. no vacuum advance in it.

[Tuner]

Turn the pink cam on the primary upside down with the screw in the # 2 hole in the lever and # 1 hole in the cam. You will need to select best fit to the pump lever and also adjust the pump lever, so the 'knee' instead of the radius pushes the pump lever. This roughly acts similar as the blue pump cam and will give much more pump shot just off idle. Be sure to adjust the spring so there is preload on the diaphragm, no slack when the throttle is closed and it is not bottomed out at WOT.

If I have a pink secondary cam can I do the same and flip it over and use the #2 hole in the lever and the #1 hole in the cam? If not, what's the best way to address the secondary pump cam?


Thanks

Yes.

You may (in fact probably will) need to adjust (bend) the angle of the lever over the cam and adjust the spring to achieve the desired full stroke of the pump diaphragm. Paying attention to the pump lever and spring adjustment is necessary with any pump cam change.

Thanks guys, the idle is set at 700 rpm and it idles beautiful and if you shut it off hot it will start with the bump of the key. It has a very small cam in it and a wiend stealth intake on it. The idle timing right now is 14 degrees and it has a mechanical msd dist. no vacuum advance in it.

Have you changed the advance springs? The original springs in MSD distributors give a conservative advance rate so they comply with California emissions requirements. If you have the small parts kit that comes with a new distributor, install the black bushing and two blue springs. Do not use mismatched springs, always use identical springs in GM Delco type distributors, MSD, HEI, GM points, etc.

[BLstangin]

Yes tuner that is how the advance is setup on the dist, I believe its supposed to give me 18 degrees of advance but according to my timing light its giving me right at 20. Also I have the blue springs in it so the curve comes in quick. Thanks for the tips.

[pamotorman]

make sure the idle transfer slots are not open too far as this will cause a stumble right off of idle because there is not enough fuel left in the circuit to cover the time it takes for the fuel from the squirters to reach the intake manifold

[BLstangin]

Just thought I would give an update, I flipped the pink primary acc pump cam as tuner suggested and re adjusted and it completely fixed the issue it runs great now. Thanks for the tips guys!