Motor is a 460 with an Edelbrock Performer 460, Holley 770 street avenger with vacuum secondaries and everything else is stock to my knowledge.

Just rebuilt the carb due to letting the fuel tank run empty and the fuel filter was full of dirt and it started to run like crap.

I have the mixture screws set at 1.5 turns out, idle at 750 rpm, timing at 12* btdc and I put a vacuum gauge on it today and got 19" of vacuum at idle in park and 15.5" in gear.

My problem is it sputters under light throttle from idle up to about 1500 rpm and it has a split second dead spot from idle under WOT in park. But, if I give it light throttle from say 900 rpm there is no sputter and no dead spot under WOT, only from idle. It has #70 primary main jets and #75 secondary main jets. When I rebuilt it I put a 6.5" power valve back in it but the rebuild kit came with a 10.5" so I tried that with little to no improvement.
I heard there are two ways of determining what power valve you need. 1. Divide vacuum reading by two... 2. Subtract two from vacuum reading.

Questions:
Do I need a power valve that's closest to 7.75" without going over? Or one that closest to 13.5 without going over?
Would changing the main jets help?
Or does the problem sound like the accelerator pump? Which I know nothing about...

 

Accelerator pump. It's job is to give a boost of fuel at request to let the carbs inner workings get up to speed. As for all the other stuff, i have no idea.

Here is a link to adjusting your accelerator pumps and power valve equation. Probably some other stuff too.

Holley 770 Street Avenger Stumble off idle-Help!!

 

Tell me how you set the idle mixture?

 

Sounds exactly like the problem I'm having. I have been reading since I posted this and I think I've narrowed it down to the accelerator pump, cam, or or squirter size.

By getting the highest reading on the vacuum gauge.

 

Short answer, I assume in park/neutral and didn't richen for response? I ask these things for a reason.

Idle needs to be set in gear with an automatic. Setting it in park often results in a lean idle and transfer when in gear.

Idle is also transfer fuel, the fuel that is needed in between idle and when there is enough throttle and RPM for the primaries to kick in is provided by the transfer slot and accelerator pump. The really pump only comes into play with a large throttle application. The transfer slot is fed by the idle circuit. So if Idle is lean so is transfer. This causes a off idle low throttle stumbles.

So adjust for lowest fuel with highest vacuum in gear. Then test tip in response, if stumbles turn each 1/4 out, test, repeat. Generally good response is found about 1/2 turn out from highest vacuum.



Power valve is a stop gap, with that carb you should only need a low PV, so if it's not already in the 6.5 is better.

Accelerator pump, at idle there should be a slight gap(piece of paper) between the pump arm and head of the adjuster. Other then that unless going to a level of tuning your not ready for as long as it squirts it works.

I don't know what you mean by "it has a split second dead spot from idle under WOT in park" Are you saying that at idle if you quickly go to WOT it stumbles?

Are you using the vacuum advance, ported or manifold?

 

I set the idle and mixture screws in park.

What do you mean by "lowest fuel"?

Are you saying put the 6.5" PV back in and not a 7.5"?

The split second dead spot is like a flat spot or like it falls on it's face then comes alive from idle to WOT if that makes sense.

Vacuum advance is ported

 

Lowest fuel = least fuel, screws the most in.


If the 7.5 is in it now I don't see it worth bothering with right now, likely the 6.5 is better but not enough info to say for sure.


There is a big gap between idle and wot, so no, depends on how fast you move the throttle.

 

Power valve selection - the varying advice seems to have something to do with whether stock (high vacuum #s at idle and cruise) or, a rumpety-rump hot cam motor that will barely "idle" except at 1500 rpm anyway some of these might top out at 10" or 11" Hg. A regular street driven motor otoh, it's important not to have the power valve open constantly at highway cruise. Mainly the racing motors want to see a power valve that isn't fluttering in and out at idle.

 

I have nearly the same setup and close to the same jetting and settings. One exception is I run manifold vacuum advance (idles better). I had the same stumble problem you describe, after a recent rebuild. Installing one size larger accelerator pump squirter nozzle solved the problem. Also tried 2 sizes larger and it was worse. If you do this change be careful when tightening the screw, there are only partial threads to allow fuel flow and they are very easy to strip.

One thing to make sure of is that the accelerator pump linakage is adjusted right, it should shoot immedetly with the slightest movement of the throttle from closed. Also make sure it has some free play at wide open, Check specs I think it's 0.020" freeplay at WOT. After this is verified try the larger squirter.

When you open the throttle blades the engine goes lean. The accelerator pump compensates by shooting a shot of fuel in at the moment the throttle blades move.

You have plenty of vacuum so I would disregard the power valve change.

 

I never have luck with the mixture screws at exactly one and a half turn out..Its the best ballpark but it's hardly ever accurate, what I do is tighten the screws all the way in (lowest fuel as mentioned above) until the motor starts running choppy then back em out until she's purring.. Its more about hearing and listening to the motor.

Your symptoms do sound like what the problem would be on a motor craft.. The accelerator pump or squirters

 

One and a half turns is the starting point.
Then tune from there.
Your method of leaning until chop then backing out till it purrs is correct.
Power valve has nothing to do with your stumble off idle.
Tune the accelerator pump system.
I'll shut up.
You are getting good advice.

 

In my experience here I find more comfort when 15 or 20 people have the same general answer

 

Did it stumble before it was rebuilt?


If yes, then you need to look at the accel pump cam, assuming the rest of the system is correct and the squirters are shooting fuel correctly. (hard fast stream)

If the answer is no, then you need to pull it apart and clean it again.



PV number I use is vacuum reading in park/neutral and then divide by 2 and subtract 1. So 19" vacuum is 8.5" PV.

Adjusting the air/fuel idle screws to the highest vacuum amount is the same as listening to what the engine likes...you are good there.

 

Yes, it stumbled before the rebuild.

I am getting different answers about the power valve (which I know isn't my problem. And mixed answers about setting the mixture screws.

Do I do it in park or drive? It's still running very rich.

My accelerator pump linkage is tuned. But I'm still having the stumble. I'm going to try different cams and higher squirters.

 

start with the pump cam only...if you can't tune it out with that then add squirter

if it is rich at idle (in park or drive) then lean it out!

The PV is not your problem - correct...you are close enough for now