Accelerator pump shot?

[junebug]

Holley 600 on a 302 with performer intake and rv cam. NV3550 35’s and 4:86 gears. Carb runs perfect at idle and wide open but bogs at beginning of acceleration. Has a #31 in it now. What size are you guys finding that works well? Figured I would try two sizes up.


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[byson1]

I can't help on the size but you can easily see if the pump is working (or if there is a restriction) by looking down the carb and manually turning the accelerator rod. Make sure the engine is not running. There should be two equal streams of fuel flowing.

 

[SHX669]

Are you talking light acceleration or medium or stomp the pedal to the floor acceleration?
Another aspect of the "accelerator pump thing " is WHEN the pump squirts fuel .
Holley has some great videos and tech help on the WWW that explain the relationship between the different ' circuits ".
A couple of issues I've seen are : the accelerator pump cam arm adjusted incorrectly{ too far down } and consequently reducing the "pump shot " - lever down on the diaphragm and reducing it's volume . Another issue that can cause an off idle stumble on Holley carbs is the throttle plate /idle speed adjustment . There is a slot { transfer slot } in the primary barrel side that controls the transition from idle to light /medium acceleration . When properly adjusted this slot should form a square - as long as it is wide - when you're at idle . When adjusted this way both of my Holleys idle at about 750rpm. I usually adjust this { with the idle speed screw } before I install the carb , then mark the position of the screw with a paint pen/marker so when I start messing with stuff I can always go back to where it belongs.

 

[rmk57]

Could also play around with ignition timing. With a cam change that can change the response from off idle.

 

[B RON CO]

Hi, X2 on the Holley website. The accelerator pump lever has an adjustment and should just be touching the pump arm so as soon as you give it gas, you activate the accelerator pump, and make sure it comes off when you are are idle. Good luck

 

[blubuckaroo]

You mentioned an RV cam in your description. With a good RV cam you get a longer vacuum curve. That can require a stiffer power valve on a Holley or a stiffer metering rod spring on an Edelbrock carb.

 

[gr8scott]

Also, anytime you adjust the curb idle, the spring will need to be adjusted so its just touching the pump arm as
B RON CO stated above. If there is any gap between the spring and pump arm you'll get that off-idle acceleration bog.

 

[pcf_mark]

I am assuming from your description that the issue only occurs at the very first degrees of pedal travel but once it happens the engine is lifeless for an instant to a few seconds and it feels like crap.

What transmission?

An .031 is pretty hefty that should cover a pretty good bog. A bog indicates lazy fuel but not lean. A pop is lean or misfire you need more fuel - usually a poorly adjusted or bad accel pump diaphragm. Please confirm you have a bog and not a distinct audible pop.
Engine off, cool engine look down the carb and make sure you get some accel pump squirt with even the slightest throttle movement. If you have this good if not google adjusting holly pump arms. If you have bog / flat spot at very early travel it is either not adjusted right or timing (see below) because the fuel is getting there and you have not opened it enough to run out of accel fuel yet (that is the cams and holes some people mentioned)


How is your vacuum advance hooked up? Ported or straight manifold vacuum? If ported I would recommend you switch it to manifold if stick shift and take it for a drive. Don't change anything but the idle speed may need to be slowed down. If that solves it then you can leave it like that or adjust your base or mechanical advance. Most tip in bogs are timing not fuel.

Take it for a test drive with a vacuum gauge hooked up so you can read it (or have a passenger read it) with your cruising vacuum. This will tell you how to set up your power valve. If the PV is late coming on you can get soft pick up not matter how you adjust the accel pump. If you are pulling a lot of vacuum (16 for example) and your PV is 8.5 it is not going to open until the vacuum drop to 8.5 so you are going to feel a dead spot. You would not want a 16 but a 10.5 you really notice.

 

[junebug]

Ok... so drove it after installing a 35 nozzle. It woke it up a lot. I still have a hesitation from off idle but then it hits awesome. My ignition is a Mallory unilite and operated fine with my last carb so I think my timing is good. I will try to adjust the arm and go from there. I will get this thing right....


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[B RON CO]

Hi, yes the whole package is sweet. Timing, carb, more timing, more carb.
You are at the fine tuning stage.
Only make one adjustment at a time or you will not know if it helps or hurts.
If anything else, like vacuum advance or vacuum leak is an issue, you won't get any further on down the line.
As good as it can possibly get is a realistic goal.
Good luck

 

[pcf_mark]

I'm not sure why you changed carbs but your timing may have been fine but now it could be off from what you need now with your new combo.

Good news the .035 worked - that is a big mutha. You may by plenty rich later in the travel of the pump. You may get dark smoke on throttle roll.

Pushing timing around could let you use less squirter which is throwing fuel out every time you touch the gas.