top of page

Accelerator pumps and more

I’m offshore currently, I will add videos and photos to this article at a later date to help show and explain things. For the time being, it is just text.

One of the most common complaints seen online regarding a carburetor is the dreaded off idle stumble.

Typically the response is 100 people telling you to change pump cams or screw the pump adjustment nut one way or another. Another handful of people say to change the pump nozzle to a larger size, or even use a 50cc pump.

Typically, the pump settings are already fine, and larger is not always better, but there are a dozen aspects to this issue worth going over.

Like anything with a Holley, there is almost never one single factor that can solve an issue, everything needs to be working together for the issue to be resolved.

First, lets understand the function:

Engine is idling in gear or moving at a slow speed. The driver snaps the throttle open and the engine does not respond for a few seconds, later it accelerates normally.

What is happening in the intake is that when the throttle snaps open rapidly, the vacuum drops just as rapidly.

As engine speed accelerates, vacuum builds.

Two issues are caused here. One is that once vacuum drops, fuel can fall out of suspension. This is a basic principle based around boiling and vapor point at certain temps in a vacuum. Reduce vacuum and fuel will be more likely to become a liquid vs a vapor. The resulting lean spike reduces power when it is needed most.

The second reason is the pull on the fuel restrictions. If vacuum goes from 18in to 5in, less pull is seen at the fuel restriction, this compounds the above lean issue.

Holley and most other automotive carburetor manufacturers added an accelerator pump system to address this issue.

The function is that when the throttle is opened rapidly, fuel is emitted by mechanical means into the air rushing through the venturi. This added fuel is sprayed directly onto the booster and is broken up on them. The finer mist produced is further shredded in the rushing airstream.

By adding this auxiliary shot of fuel, the small lean/low vac period is covered so the engine can accelerate while the carburetor fluid flow recovers.

System breakdown on a 1960s Ford Holley:

From the throttle shaft to the end the mechanism goes like this.

Throttle has a pump cam.

As throttle is opened, throttle shaft rotates.

As throttle shaft rotates, cam profile changes providing lift to the pump lever.

Pump lever on the throttle shaft end rises, this causes the lever to pivot on the pin in the baseplate.

This turns the rising action into pushing down.

The pump lever pushes agains the pump cover arm.

The pump cover arm pushes up against the pump diaphragm.

This forces fuel to be pushed out of the pump housing and into the metering block.

The fuel travels through the metering block into the main body.

From the main body the fuel emerges from a pump nozzle and sprays fuel into the venturi.

Upon release of the throttle, the pump spring pushes down on the pump diaphragm which refills it with fuel, ready for the next use.

Notes worth mentioning:

Every 1960s Ford Holley used a white pump cam.

The nozzles used were from .021-.029 on all of these carbs.

All of these carbs came with a 30cc pump.

Ford nor Holley ever saw the need to diverge from this pump cam and the majority of their carbs had a .025 nozzle.

This was for every carb, be it a 390 engine in a grocery getter or a factory race carb.

Something to consider. Yes there may be a circumstance where the need for a larger nozzle or different cam is required, but just think about the factory high riser carbs and the radical engines that used a .025 pump nozzle before you decide a .038 is the answer to all of your problems.

A simple lesson that is often misunderstood.

The pump diaphragm is of a given size and contains a specific volume. This volume does not change.

As such, a .021 and a .038 nozzle fire the exact same amount of fuel for one Opening of the throttle from idle to WOT.

The difference between the nozzle is the duration of the pump shot.

The .021 nozzle fires less fuel right off, but the duration is longer.

The .038 nozzle fires more fuel instantly, but the shot duration is considerably shorter.

Often the complaint is “when I stab the throttle, the engine accelerates, slows, accelerates.”

Typically the advice is to use a larger nozzle, and to use a more aggressive cam which pushes faster on the pump. As you can imagine, this is the exact opposite of what should be done.

Pump settings on their own are fairly simple.

Let us assume you are using the stock white cam in the #1 position, a 30cc pump, and a .025 nozzle.

The simple answer is to set pump with some preload at idle, with no binding at wide open throttle.

The book spec for WOT is to have .015 play between pump screw and pump cover level.

I don’t get quite so specific for a basic setting.

Pump screw adjuster for me is not really an adjuster, I set it to a specific height which preloads the spring a certain amount.

The pump screw adjuster is three parts:

  1. Top screw (male)

  2. Spring

  3. Bottom piece (female)

Ford used two different bottom pieces. A short and a longer. To attain the same spring preload, the overall spring length when assembled is 1/2inch for the shorter bottom piece and 5/8 for the longer.

I find using the #2 cam position allows me to get more action and overall movement of the pump lever as the lever rests more on the “heel” of the white cam.

This may create a dynamic where you have too much or too little pump preload.

By taking a small adjustable or channel locks, I slightly bend the pump lever where it contacts the pump cam. By doing so, I can get the least amount of preload and the greatest amount of level movement.

The end result is attaining the greatest volume moved out of the pump cam and through the nozzle.

For the experimental types who aim for perfection, you can add some slight bends or bows on the lever where it touches to cam to get 100% of the pump diaphragm to evacuate the housing.

It’s fun to play with, but rarely essential, so don’t feel like you are neglecting something by not achieving this.

Causes, fixes, things to think about assuming you have done all of the above:

-Distributor advance.

If the mechanism is sticky and not smooth, or if the advance setup is sluggish, this needs to be addressed. A weak advance of timing can contribute to a bog. As engine speed increases, timing plays a critical role in firing the spark for the accelerating engine. Greater piston speeds require more advance to make the most of the air fuel charge and resulting burn rate. The same can be said for loads and rich/lean conditions. These all have different timing requirements.

If you are the type who wants it simply done, contact a distributor expert who can recurve the advance setup for you.

If you are the do it yourself type, first plot out the current advance curve with a timing gun and notepad. For every 500rpm, note the advance and make a chart. This will give you a baseline for future experimentation.


Many factors here.

Weight of vehicle.

Rear end, transmission gearing, tire size.

Automatic torque converter stall speed and converter design.

If you have a 5,000lb pickup truck with 3.00:1 rear gearing, 33inch tires, and a stock stall torque converter, trying to get rid of a bog will be much more difficult than a lighter, high geared vehicle, with a 3,000rpm stall.

With my Galaxie, although heavy, I have 4.30:1 rear gears, 28inch tires, and a fairly high stall speed. It is nearly impossible for me to get the engine to have an off idle bog. The engine simply doesn’t load until rpms are high enough that we are past the dead spot.

With the aforementioned pickup truck, if you stab the throttle, the truck is instantly loaded heavily at low rpm.

My car’s engine in the same scenario is at 3,500rpm before it is heavily loaded and thus much closer to it’s torque peak.

The same sort of dynamic plays with manual vs automatic transmissions, they will load differently, and it is something to think about.

Can this be changed just with the carb? Yes, to a point. You can try to tune it out, but if extreme off the line performance is desired, it is about the whole package, not simply a nozzle change.

-Impact of Float height

Float height? Why would that be an issue?

The main well activates as rpms climb and the throttle opens. Bleed air lightens the fuel in the metering block to make it flow easier. Float height has a direct and profound impact on how soon the mains start.

The reason for this is as I’ve explained in the past, the air flowing through a Venturi creates a depression. This depression pulls on the fuel. If the fuel is lower, greater depression is needed and it takes longer to pull it out of the nozzle.

If fuel level is higher, the fuel is easier and faster to pull.

Simple as that.

Holley specifies a dry float level at 5psi for most every carb from the 1960s. You recheck this via the sight plug.

These get you right in the neighborhood for the fuel level they designed around. That said, small variations may be used to impact change on when fuel emerges from the nozzle.

-Float drop, fuel pressure

A common issue.

Customer: “Car takes off super hard, lays down and almost dies, slowly accelerates up to speed. I have the fuel set at the window and have 9psi fuel pressure, so there is plenty of fuel.”

Me: “Run 5psi and reset float level to window.”

Customer: “But that is less fuel!”

Ok, lets unpack this one. I won’t bother explaining the whole pressure vs flow dynamic, I’ve found people that understand it don’t need to hear it, and people that need to hear it, will not understand it.

Inside the bowl, the fuel level is set by a combination of fuel pressure and float height.

The float opposes fuel pressure to seal the needle against it’s seat.

If we use the stock 5psi fuel pressure setting, you typically will have the float set nearly to the top of the bowl. It doesn’t require much buoyancy to seal against incoming pressure.

Fuel is at the bottom of the sight plug.

With this setting, under hard acceleration, the fuel in the bowl begins to be used. The float drops and the needle and seat open. Due to the high float setting, it can drop to the bottom of the bowl if needed, providing a free and clear path for fuel to emerged from the needle seat.

With float dropped low, it does not interfere or restrict fuel flow.

Now, lets imagine the same scenario at 9psi.

In order to get the needle to seat and counter the higher incoming pressure, we have no choice but to screw the needle/seat assembly further down. This pushes the float deeper into the fuel so more of it is submerged. The extra buoyancy of the entire float being below fuel level aids in sealing against the high pressure.

Open the sight plug and sure enough. Fuel level is right at the bottom of the plug.

Good right?

Ehhh, not really.

Here is what happens.

The float is already very low in the bowl. When we accelerate hard, the fuel rapidly gets depleted, and incoming fuel needs to replace it. The problem is, with the float already low in the bowl, it cannot drop but a small amount before it is against the floor.

Looking at the needle, it is still very near the opening at the seat and is acting as a restriction to incoming fuel.

When we take off hard, we go like hell until incoming fuel runs out and cannot be replaced fast enough.

I hope you can understand and visualize why in this case MORE fuel pressure can actually provide LESS fuel entering the bowl.

-Idle overly lean

A common issue is running a lean ragged idle. The problem itself is with people tuning “by the book.”

What I mean, most manuals written and online show finding perfection in idle tuning by setting initial advance and mixture screw settings on an engine with nothing more than a tachometer and a vacuum gauge.

Yes. This is in fact how you should do this…. With one acceptation. It needs to be done under a typical load.

This is especially critical with modern performance camshaft designs.

Anyone who has had the opportunity to watch an A/F meter, has seen that if you set a hot idling engine to a fairly steady a/f ratio and put the engine in gear, the a/f ratio will swing lean.

Add electric fans, air conditioning, etc and the engine is practically stumbling lean.

By setting the engine to the smoothest idle at no load, you are running the ragged edge when loaded.

I tune for the driving I do.

In this case, if you have a/c, crank it up. If you have electric fans, fuel pumps, lights, etc, turn them all on. Put the engine in gear. Obviously safety is an issue, block the wheels and have an assistant foot break while you set for the smoothest idle at full load.

The same dynamic is played out with idle speed.

People want the lowest idle speed instead of the best idle speed.

Set the idle speed for full load usage.

Sorry if the added 200rpms makes the cam sounds less cool…. Best performance is way cooler than stalling out in a parking lot.

Why does this impact off idle acceleration?

If the engine is already on the edge, any subtle changes can quickly put it well outside of the range of working well.

If the engine is already running smooth, with plenty of fuel, rapid changes tend to have a lessened impact.

-Impact of bleed air

On original factory Ford performance carbs this is rarely an issue, but worthy of a mention. Emulsion air and large high speed air bleeds may make a ton of power on a dyno by aerating and moving the most fuel, but often they can make the transition to the main circuit a little grumpy.

By adding some air to the main well, we aerate the fuel. This makes it flow easier due to lessened weight. It also helps pre-atomize the fuel due to a more gaseous nature.

Adding an excessive amount of bleed air in some cases will cause the air to pool, so when the main boosters begin to flow, we get a shot of fuel/air/fuel/air instead of a nice homogeneous mixture.

On a dyno or a drag strip, this may never be noticed. But at certain slower cruising speeds, it can be pretty obvious.

Again, not an issue with factory carbs, but many “race” carbs which are designed for high speed operation can have this issue.

-Transfer slot relationship to main circuit

Everyone has seen the “proper” t slot exposure and no doubt has set up a carb based on that.

The reason to not overly expose the tslot is so you have plenty of throttle movement prior to the main well activating.

Many carbs were designed outside of this specification, Cobrajets being the most obvious. To idle a CJ carb, you need a considerable amount of t slot exposure by design. The main circuit however was designed to accommodate this, so it is not an issue. CJ boosters and bleed arrangement make for a very short idle and early main activation.

As always, finding the balance is key.

In the case of many modern carbs, I often think the manufacturer pulled ideas from different places without fully understanding why.

As such, we have a short lean idle and a delayed main circuit. This is hard to fix.

Holleys Street avenger carbs seem to yield the most complaints. There is no reason they cannot be used, but some tricks like raising float level and richening idle circuit become all the more critical to help these circuits meet and even overlap a bit.

-Power valve

Oh lord. Not the power valve. The cause of all carburetor woes! 😂

Alrighty, here goes. First. First dang thing to understand, a PV is rated at the opening point. It is closed until: 3.5, 4.5, 6.5, 8.5, 10.5, etc. the PV feeds the main well only. It does not impact idle. If you find it does impact idle it is because you have a leak internally.

The common wisdom is to use a power valve that opens at a low vacuum rating.

Maybe this works sometimes. I prefer to ignore it. The purpose of the PV is to open when you need fuel added to the main well. The typical usage is as such:

Cruising. While cruising you are at a high manifold vacuum situation, the fuel is being pulled primarily from the main jets. Due to the manifold pull, the jets are being pulled on quite hard, good smooth fuel flow.

Now you need to accelerate rapidly, so you depress the throttle. Two things happen right away. Manifold vacuum drops, and thus fuel isn’t pulled as hard from the boosters (momentarily).

It is at this exact point the power valve needs to add fuel. The added fuel compensates for the loss of pull, and also as load increases a richer condition is required. To this day, I’m still not certain how idle vacuum plays into figuring this…

The best way to determine this is to drive in this load/acceleration situation with a vacuum gauge. It will be apparent.

Many people induce a high speed loss of power here. The example is the guy who uses a 3.5 pv due to his big cam. It takes a lot of throttle to get to 3.5in vac when at speed. It may work fine for sudden WOT but it often misses the mark for lesser measures.

From the factory, Ford used 6.5 and 8.5 pv ratings in all of the carbs I have seen.

The idea was to have the leanest, cleanest cruise possible while providing auxiliary fuel early as needed. When at speed and adding power via throttle, you may never notice the a/f ration being a hair too rich, but you will certainly notice it being extremely lean.

-Vacuum secondaries

Simply put, if you are cruising, you slam the throttle open, and the engine temporarily gets weak before accelerating, the vacuum secondary spring may be too weak.

The reason for this is that the secondaries should open slowly and smoothly. You have no secondary pump shot, so the idle and main circuits need a little bit of time to activate.

It can snap open at low rpm as well, but this is less common.

Changing the spring to one slightly stiffer may solve your problem.

-Ported vs Manifold vac

I hate to even mention this at all. So many people seem to miss the mark on this one.

I will simply say, for the 1960s Ford performance carbs, if they used vacuum advance, it was always ported.

I am currently unaware of them ever using manifold vacuum advance.

I hope this helps you.

Maybe I mentioned something you had not thought about…. Something worth checking.


919 views0 comments

Recent Posts

See All

Torque Specifications

Just putting this info here so it is easy to find in the future. Torque specifications: Type In/lbs Fuel Bowl Screws 25-30 Secondary metering plate 12-18 Needle/seat locking screw 50-60 Center hung fl


bottom of page