Holley Off Idle Stumble

[355_79bu]

I have a Holley 4160, 1850S (earlier version with adj. floats), 600 CFM, Vac. Sec. single feed carb. Since it was new it has had an off idle stumble. I have checked, and adjusted the accelerator pump arm, and of course set the float levels, all of which did not correct the problem. I was thinking of trying a size or two bigger squirter (it currently has a 31 squirter), or maybe trying different accelerator pump cams. I have also heard of changing the accelerator pump out for the 50cc version to fix the problem. Anyone have any advice on where to start?

 

[mreese]

i would try the squiters first.

 

[bowtie81]

i use to run a 50cc in the front and 30cc in the back when i had my holley and it worked fine for me.

 

[art]

How did you set the idle mixture screws? Depending on the cam and timing, your carb may not be at the root of your troubles. To get you in the ball park, set your timing with a vaccum gauge and a tach. The higher the vaccuum, the "happier" your engine is idleing. By setting the timing first you will probably wind up lowering the idle setting which in turn, gets the throttle blades back to where they belong. Then play with the idle mixture. Many stumbles, especially in street set ups come from not enough timing at idle. If you recently did away with vaccuum advance and went to a "locked out" setup, you mave have done yourself a disservice. My setup loves initial timing. I have 18 degrees lead with a 20 degree pot for a total of 38 degrees at idle. (credit my crummy stock heads for most of that) Keep us posted.

 

[LS6 Tommy]

Sounds like a classic example of too much primary idle speed. Just to give you a really watered down explanation, when the primary idle is too high, the transfer slots in the venturii are below the throttle plates, so there's not enough fuel added as you roll it open before you go into the main circuits. What rpm do you idle at in neutral?

Tommy

 

[355_79bu]

Thanks guys, but I do not believe this is a idle circuit or timing problem. I have already have played with the idle circuits and have them perfected. The car idles at 800 in neutral, 700 in gear, and is no where near being opened up into the transfer slots (I checked this on the bench). I get good response out of the mixture screws and set them the way I have set many others, with a vacuum gauge to the highest vacuum which puts them 1.5 - 2 turns out.

As far as timing goes, I still run the vacuum advance, and have had tried several different settings between 8-18 deg initial. I run N20, so I have really played around with timing. The car ended up liking the initial around 14 deg. The stumble never went away. No change at all, no matter what timing I threw at it.

My brother has the carb off his race car while doing an engine swap. He has a 33 squirter in his carb. I'm going to give it a try and see how my car likes it. It seems to be the easiest place to start.

 

[mreese]

Thanks guys, but I do not believe this is a idle circuit or timing problem. I have already have played with the idle circuits and have them perfected. The car idles at 800 in neutral, 700 in gear, and is no where near being opened up into the transfer slots (I checked this on the bench). I get good response out of the mixture screws and set them the way I have set many others, with a vacuum gauge to the highest vacuum which puts them 1.5 - 2 turns out.

As far as timing goes, I still run the vacuum advance, and have had tried several different settings between 8-18 deg initial. I run N20, so I have really played around with timing. The car ended up liking the initial around 14 deg. The stumble never went away. No change at all, no matter what timing I threw at it.

My brother has the carb off his race car while doing an engine swap. He has a 33 squirter in his carb. I'm going to give it a try and see how my car likes it. It seems to be the easiest place to start.

try capping off the vacuum advance all of them i
have had didn't like it.

 

[purple barney]

with the carb being a vaccum secondary try chainging the spring rate in the vaccum secondary housing. the best part is you have the test drive the car to get the right spring tension heres a link to the part http://store.summitracing.com/partdetail.asp?part=HLY-20-13

 

[MalibuMan1979]

this might be a given ,but you didn't mention if you tried a bigger primary jet size. also, with a holley you can richen off idle accell by setting your primary float higher than the lower edge of the sight window.

 

[7d8ss]

Personally, I would try a smaller squirter. If you can get a #28, try that. Then I would consider a different pump cam. ~ Jon