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Holley still stumbles on acceleration

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matt2491

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Discussion starter · #1 ·
Trying to figure out my lean stumble when flooring it quickly. The engine is a 350 with Vortec heads, Comp 268H cam, air-gap intake, and new Holley 4160 600cfm (list # 0-80457s). I've been tuning the car with a AEM wideband, which by the way has proven to be incredibly useful. I feel like I have X-ray vision! :D At this point, I have the idle circuit and main jets tuned in pretty well. I also stepped up from a 6.5 to an 8.5 power valve since my cam pulls more vacuum, around 10.5 at idle in gear.

When I'm driving around, the carb performs just fine. Nice smooth power at all speeds. I can also give it moderate throttle and it'll accelerate smoothly. However, when I give it a quick stab it stumbles for a split second then hooks. The wideband pegs lean for this split second. This happens at any speed, not just off idle. The problem is, I've been gradually going up on squirter sizes and am currently at .035. I can't imagine my small block requiring more than that, right? Is this a pump cam issue maybe? I do have it adjusted properly, as in no slack at rest, and it doesn't bottom out the pump at WOT.

What do you think? Keep going up on nozzle size or mess with the cam?
 
#2 ·
I have the most aggressive pump cam on the hottest setting in my 600cfm 80547...with similar engine and cam. Also, I made sure the "lash" (0.015") on the accelerator pump spring was right on. I did these things to fix an off-cruise bog that turned out to be something else, but it is definitely more responsive now, too.
 
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#3 ·
Put it back to the 31 shooter and see if it gets better or worse. If it's worse, richen up the idle mix screws a little and see if that makes it better or worse. It's just going to be a lot of try this and that to see what it wants. Change one thing at a time so you know what's working and what isn't. Also, don't get too caught up in trying to get the perfect numbers on the wideband. Your engine might not like your target number so just give it what it wants, use the wideband to see changes made.
 
#4 ·
try the #2 setting on the acc. pump cam.
 
#6 ·
How much initial timing are you running? Sometimes just a couple degrees will help, but it requires recurving to limit total timing. Add a couple degrees and see if it responds, if it does readjust the distributer curve to suit. I would expect 16 to 20Ëš initial timing with your setup, with vortec heads total around 32-34Ëš.
 
#7 ·
Your 600 is vacuum secondary. Most likely is the secondary is opening too quick and it's gulping too much air and not enough fuel. Check if you have a check ball in the diaphram housing.With the check ball the secondary will open slower.
 
#9 ·
You mention the carb runs very smooth if your easy on the gas and bog if you punch the gas? That is the secondary opening too quick then it closes..
 
Discussion starter · #10 ·
Thanks for the tip Louis. I just tried disconnecting the secondaries and tying them back. It made no difference on the stumble. So I'd have to agree with you Jake, I think Holley sets up the secondaries to come in a little late if anything.

So now that we know its not the secondaries causing the stumble, I tried installing the blue cam (most aggressive) in conjunction with the .035 nozzle. That lessened the stumble, but under WOT stabs its definitely still there. I've also been using the #2 hole exclusively on cam. #1 doesn't let the arm contact the profile correctly in my case.

My idle mixture screws are almost exactly 1.5 turns out, which gives me an average 13.5:1 AFR in gear at idle. Holley nailed the balance for my engine. The carb originally had #64 jets, which was a bit too lean, so I went up to #66 jets which the engine seems to like. My highway cruise richened up from 16.5 to 15.5.

Also, in terms of the history of what I've done, I started out with the carb in its factory settings (.031 and orange cam). It was a dog off the line, so the first thing I did was swap out the orange cam for the blue cam. That helped a lot, but didn't solve it. So then I thought a .028 nozzle might help extend the shot, but that didn't improve anything. Now I'm on a .035 nozzle, first with the green cam, which was better overall, but with the blue cam is best. Stumble is still there though! :confused:
 
#11 ·
I read in another post where it was mention the spring that sits on accelerator pump arm was weak, causing not to pump enough fuel.
try it.
 
#12 ·
Have you tried not going by the meter and just adding 1/2 turn to both idle screws and bringing the floats up till they just drip out the threads slightly?

Each easily changed back.

I'd probably also step up slightly on the power valve like from 6.5 to 8.5 to try and add a little more fuel during throttle roll in. PV only adds fuel to the mains when the circuit vacuum drops below the number the PV is rated. If idle speed is fairly low, there's not enough signal to start fuel through that circuit.

If it pops,it's most always air and lean.

Randy
 
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Discussion starter · #15 · (Edited)
I timed it by ear and by vacuum. Its advanced as far as it'll go to maintain the highest vacuum reading and ease-of-starting.

EDIT: Just want to clarify that this is not a light off-idle stumble, only when putting the pedal down fast.
 
#16 ·
I would suggest getting a timing light and dial im the timing first, and then start tuning the carb.
 
Discussion starter · #17 ·
Wouldn't timing it by vacuum be more accurate for any given engine? In the past I've used timing lights to set the timing to what people and/or manuals have said it should be at. But then I've always been able to get it running even better by re-adjusting it by vacuum. Just my experience.
 
#18 ·
matt2491 said:
I timed it by ear and by vacuum. Its advanced as far as it'll go to maintain the highest vacuum reading and ease-of-starting.

EDIT: Just want to clarify that this is not a light off-idle stumble, only when putting the pedal down fast.
Click to expand...
Have you recently changed the distributor from a points-type to a plug -n-play HEI type by any chance?

If you wire the hot from the HEI to the cotton covered wire that was used for the points type, there won't be enough voltage and it will pop.

Randy
 
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Discussion starter · #19 ·
Thanks for the insight Randy, but the car has had an HEI all along. It receives its power through an American Autowire classic update kit and seems to work well.

Idea that just dawned on me: Could my 1" four-hole phenolic spacer be the cause of the stumble?
 
#20 ·
Go you have a friend that you can borrow a carb from? That carb should be just fine right out of the box. I think you said earlier you don't use vacuum advance. Does it do okay slowly accelerating vs. mashing the pedal? Are the floats set right? That can impact the transition circuit. Does it stumble from say 2500rpm when you are in the jets or just from 2000 or under? As already said, if the carb has been apart and secondary vacuum messed with that can be a big problem. They work by engine load and spring tension. How far out are the idle screws? Did this carb ever work fine before?
 
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