Slick_Sammy

holley 4150 double pumper (4779-6 model). having trouble getting rid of stumble when secondaries crack open. i've tried all the basic easy stuff-- change pump cams, squirters, adjust throttle blades, play with idle mixture screws. I think something is wrong with my idle transfer circuit so I think here is where I need to focus the tuning.... My idle mixture screws weren't doing much of anything when turned in or out. OK, so I cracked open my secondaries so the transfer slots are exposed just a little (about .015 -.020) and brought my primaries down so the slots were only exposed about .040 @ 1000 rpm idle. This didn't make much difference. When I turn one of my idle mixture (4 corners) screws in ALL THE WAY, the RPM and vacuum goes up a good bit. it should stumble and go down as it leans out right? so this tells us it's still getting fuel from somewhere it shouldn't right?

Any ideas of stuff to try from here? this is a 400 small block (destroked to 377 cu). Motown 220 heads, mild cam (too mild really for the heads which is why i get low vacuum). Timing locked at 36* but I'm going to try going up to 40. This is a drag car (footbrake). Right now I want to try to focus on getting my idle circuits right. I think that my in itself clear up my stumble when secondaries open. Any help appreciated!

Damon

It wants what it wants. If it wants a leaner idle then screwing them in will raise the idle. When you go too far lean the idle will flatten out and drop. You should adjust them evenly and in small increments. Don't have one out 2 turns and another one almost fully closed, for example.

Regardless, if you've got a stumble when you open the secondaries your problem has nothing to do with the idle circuits. By the time you're at half throttle and above the idle circuits are basically meaningless.

Slick_Sammy

i had put them all out 1.5 turns to start off with. i turned one in all the way and idle went up quite a bit. i thought when it got all the way in the idle should go down at least some even if it was just one? so you're saying turn them all in maybe to 3/4th turn out and see what happens then? i think i know the right proceedure but my idle screws don't seem to be reacting the way they should.

angel71rs

.040 is still too much on the slots. Have you tried drilling the butterflies for extra air so you could close them up more? .025 is about where they should be. Too much slot exposure can leave too little fuel in the circuit to make the transition to the main feed smoothly. Maybe this is your stumble?

Are you running a power valve with a low enough rating so it doesn't try to open at idle? Blown valves don't happen too much any more since Holley started making carbs with protectors.

You could have 2 different issues, something causing over rich idle, other the stumble secondary opening stumble. Is the linkage to the secondaries giving you a pump shot as soon as they start to open?

Slick_Sammy

i thought .040 - .060 was acceptable on the primaries. ok. haven't drilled holes yet... was trying to avoid that if possible. it may be the next step though.

i blocked my power valve off and went up 8 on the primary jets to make up for it. same thing before that with a power valve.

pump shot is adjusted correctly . i may have too MUCH pump shot though as the Pink cam is in front and back. i'm going to go down on the squirter slightly in the back and maybe change pump cams out for orange which are a little more aggressive than stock white cams.

i do think i have 2 different issues.

Slick_Sammy

i think i've fixed the idle issue now. i can now kill the motor with the idle screws - yay! but still have the stumble.... any ideas on that? it happens right when the secondaries open. if i stab the throttle too quick it will die. a little less quick stab and it will hesitate bad when secondaries open then rev up quick like it should after the hesitation. it acts like a momentary lean spot but i have tried from 31- 35 squirters and pink-orange cams with no luck yet.

Damon

Is the secondary acc pump actually working? It should start feeding fuel the instant the secondaries start to open. There's a bunch of things that can prevent that from happening- check valves and clogged up passages being the usual culprits.

The squirter sizes you have tried should be plenty big- that's not your problem.

Also make sure your secondary float setting is adjusted properly.

Last, if your carb came stock with the larger 50cc acc pump on the secondary side you can use the BIG purple cam- the factory often does.