Carb Class: Giving The Bog The Boot

By: Quick Fuel Technology05/18/2010 < Back to Blog Home

Q: I can’t get the “Bog” out of my mechanical secondary carburetor what’s wrong and how can I fix it?

The sole purpose of the accelerator pump system is to provide a combustible air/fuel ratio during hard acceleration, especially from idle, for the time it takes the main metering system, via the booster venturi, to respond to the increase in air velocity. When you mash the throttle opening the throttle plates, air responds instantaneously to the pressure drop (vacuum) generated by the engine through the intake manifold. Fuel on the other hand is too heavy and cannot respond as the air does. Assisting the engine during this transition carburetor engineers developed a hydraulic fuel delivery system to push fuel into the slug of incoming air to make a combustible mixture.

The accelerator pump system is simply a mechanical fuel injector that provides enough fuel for a brief period of time for the engine to respond smoothly to the heavy wide open throttle demand. Its purpose is oh so simple and its function is oh so critical. And, when it isn’t working right, the dreaded “Bog” and the always dreaded backfire through the carburetor are effects of an accelerator pump system that isn’t equal to the task. The “Bog” simply means the engine is not receiving enough fuel when you go to wide open throttle. While there are other issues that may cause similar reactions within the carburetor, often the problem can be traced back to the accelerator pump system.

Incorrect adjustment is the typical culprit. The accelerator pump function, as mentioned, is based on hydraulic principles. Therefore, it is essential that the accelerator pump always has tension at your normal idle position. Any air gap here will delay the actuation of the accelerator pump system allowing air to rush into the engine without fuel.

The common misconception is that it is necessary to adjust the pump arm to obtain an air gap. This is correct; however that air gap is only at Wide Open Throttle (W.O.T.) not at idle. The air gap is simply to ensure additional travel in the pump arm so it does not bottom out on the fuel bowl. If it does bottom out eventually it could loosen the accelerator pump housing causing a fuel leak. Fuel leaks are something we all want to avoid.

Assuming the first adjustment is correct. Then the next step in the process is to add more fuel to the incoming air which is accomplished by increasing the size of the pump discharge nozzle or squirter. This will quite often correct the “Bog” or hesitation by richening the air/fuel mixture when the pump system is activated. If the carburetor works well everywhere else on the track then a bigger squirter might be the only change necessary.

Pump squirters are available in a wide range of sizes, the bigger the number (size) the more fuel when you accelerate. Simply remove the pump nozzle screw, lift off the old nozzle and install the new. Be sure both gaskets above and below the nozzle are replaced. If the gaskets are a different thickness the thicker goes under the nozzle and thinner one on top.

Q. How do I know if I need a bigger pump, i.e. a 50cc versus a 30cc?

A larger capacity accelerator pump achieves one thing, it extends the duration of the pump shot. A larger accelerator pump is usually required when the carburetor, intake manifold and/or cylinder head is too large for the engine and the resulting weaker signal at low engine RPMs requires more time for the booster venturi to pull fuel through the main jet. Generally, the telltale sign is good initial throttle response, but shortly thereafter the engine bogs down or flat lines and as the RPM climbs it recovers accelerating normally. That bog or flat spot is where the pump shot duration ran out and the booster venturi fuel flow alone is not enough to keep the engine running efficiently. In this case, the larger pump capacity helps the transition. While this is a more popular change among foot brake cars, the 50cc is also sometimes a necessity with large plenum 2 X 4 tunnel ram applications especially when racing on a .400 Pro Tree.

By the way for you curious types, the 30 and 50cc capacity is the volume of fuel displaced from the accelerator pump in 10 complete strokes of the pump arm. So the actual capacity is 1/10 that number or roughly 3 and 5cc respectively when opening the throttle from idle to wide open.

The larger capacity 50cc pump is easily recognizable by the large pump arm with the encapsulated spring. In addition, the 50cc pump also uses a larger pump cam either a brown color (conventional carbs) or a yellow color (model 4500).

A 50cc pump will not deliver the 50cc rated volume if any other cam is installed.