Holly Carb Adjustment - New Motor Set Up

[TX-ZACH]

Fresh off a new new motor build with ATK 351w, roller cam with Edlebrock RPM Aluminum Heads/intake, and Holly 600 Series Carb. Installer suggested a larger carb to fix and said he adjusted it as much as possible. I used the Holly carb off the 302 that was in it prior and everything I am reading says that is plenty of carb. The idle is set a bit higher to help with stall. In higher rpm range it seems fantastic and crisp. Any pointers or suggestions to educate myself would be much appreciated. It still has a 1/2 tank of 3 month old 87 octane fuel that I am trying to use before trying 93 to see if it helps.


Symptoms:
Slight Stumble putting into gear and from idle to 1/4 throttle: Feathering the throttle and slight higher rpm clears it upon take off
Exhaust smells super rich from unspent fuel

 

[El Kabong]

from idle to 1/4 throttle:

For this symptom, with the engine off, take off the air cleaner & look into the carb. As you open the throttle, see if there's a steady squirt of fuel while the throttle is moving. It should be a steady stream while the throttle is in motion. If not, there's an issue with the accelerator pump circuit.

Normally the accelerator pump provides a little extra fuel to transition from idle (or any point) to higher revs. Without that temporary extra fuel, the carb runs too lean until it catches up to the new throttle position. Without that extra squirt of fuel, you have to feather the throttle to get the revs up from idle. At higher revs it's not as noticeable.

If the accelerator pump isn't delivering that extra fuel when needed, I wonder if the adjustments he made to try to compensate are dumping too much fuel the rest of the time, causing your rich condition.

It might not be the problem, but it's easy to check.

 

[TX-ZACH]

I went out this morning before going to work and snapped a couple of pics of the carb. The accelerator pump bottom lever allowed a single piece of paper clearance and did not have any slop between spring assembly. Opened the primary and had flow of fuel upon immediate application of throttle. I started checking my plug wires while I was under the hood and noticed the passenger side (2) rear spark plug boots not fully seated and pulled right off when checking. I fully secured the boot to plug and fired it up. It appears to idle much smoother and crisp when cold start. I am going to road test it tonight when I get home.

Google Shows it is a: Holley 600 cfm Street Warrior Carburetor, Electric Choke - Shiny Finish​

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[B RON CO]

Hi, I hope the plug wire was the problem. Make sure the kickdown lever has a light spring to pull it forward. I don't see one in the picture. Good luck

 

[TX-ZACH]

I see that. I have a C4 trans and not sure if it needs it or if it was on prior to me purchasing the Bronco or if the installer removed during the install of the new 351. It was raining yesterday and no road test but will be in it today once it get daylight.

 

[BOBS 2 68S]

It looks like there are 4 throttle return springs. You should not need that many.

 

[Rustytruck]

always check and adjust the float every time you change and adjust the fuel system.
before you change anything make sure the choke system is working properly and fully warmed up.
check engine timing. now that everything is settled in hook up a vacuum gauge and adjust the idle mixture screws to the highest steady idle speed and then adjust the curb idle back to proper settings IE 700 rpm or so then check the idle mixture screws and vacuum reading again. Now go drive it. if it bogs or bucks you need to adjust the plastic accelerator cams different colors for different shots and 2 adjustment screw holes in each plastic cam for finer adjustments. then you get to decide which one works best it always better or not. after you have gone through that you get to try squirter sizes and redo the plastic cam chore again then once you get to that point you get to start worrying about jet sizes and or metering plates and booster valves. its really a sickening mess best done with a wide band meter. after 3 weekends you might get close enough to actually drive it . Patience is the key. I find most people I know do not properly set up a Holley and just quit and live with the problems previous to wide band meters you never truly knew what was going on. a 302 Holey will need a lot of changes to work properly with a 351. it may not be big changes but changes all the same for each operating circuit. start with looking up the changes Holley used for the same model carb in both the 302 and the 351. Holley had these model changes posted on line reference back when I was dealing with them 30 years ago. I don't know whats available these days. SA books had a good Holey book available that explained how the Holley worked and how to set them up. that adds another weekend to the mix.
Good luck with your adventure.