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Hesitates, or Bogs When Stepping on Gas

Carburetor technical help: hesitates, or bogs when hitting the accelerator

Why it happens:

When stepping on the gas, the engine needs a little extra gas before it gets to cruising, or power. When that extra gas is not available you will get a hesitation, or bogging. You may also hear a popping or backfire through the carburetor.
This is most likely due to to much air, or not enough fuel.
While the electrical system can cause this problem, we are going to concentrate on the fuel system only here.
One thing to keep in mind: hesitation can be a result of using a carburetor not designed for the vehicle.


Vacuum Leak

  • Check all connections that might have vacuum.
  • Also check any vacuum hoses.
  • You can use a spray carburetor cleaner and spray around the connections. If the idle smooths out, then you probably found the problem.
  • Pay special attention to the flange/manifold seal

Not Enough Fuel

  • Accelerator pump quick test if your carburetor fails the pump quick test, check out the section below
  • Check fuel pump pressure (compare the number with your motors manual)
    • General rule of thumb: 1-barrels 3.5 to 4psi, 2-barrels 4 to 4.5psi, 4-barrels 4.5 to 5.5psi
  • Check the float level (specs are provided in the kit instructions or in our rebuild manuals).
  • The needle may be sticking in the seat, not letting enough fuel in.
  • You could have plugged passages. Ethanol will leave deposits behind that cleaners and compressed air will not remove. Use a thin wire or nylon brush to clean all passages.
  • Are the idle mixture screws way out of adjustment?
  • If the carburetor hasn’t been rebuilt, then do that if you suspect dirt is your problem.

Failed the accelerator pump quick test?

The instructions below may be a bit different depending on the type of carburetor, but the general idea is the same.

Remove the carburetor from the engine.

Take the top off of the carburetor.

Fill the float bowl with a fluid like mineral spirits.

With the pump all the way down, lift it up to the top of the pump well. The pump well should fill with fluid.

  • The pump well doesn’t fill – If there is a check ball on the bottom of the pump well, then that is where the fluid fills. The suction of the pump going up and the weight of the fluid will lift the check ball and allow fuel to enter. For those that don’t use a check ball, the fuel will flow over the side of the pump well.
  • For those with the check ball check the passage from the fuel bowl to the pump well. You can do this by blowing compressed air into the pump well hole and test to see if air makes it through to the fuel bowl.

Remove the venturi.

Now with the pump well full, push the accelerator pump to the bottom. There should be fuel coming out of the main discharge.

  • No fuel means there is a blockage in the passage from the pump well to the main discharge. Clean the orifices with fine wire or nylon brushes. 

Reinstall the venturi.

Fill the pump well with fluid and push the pump all the way to the bottom again. Fuel should squirt out of the venturi. If it doesn’t, then you have a plugged venturi.

Is the venturi clear? Pump the throttle a few times to see if fluid is squirting out of the venturi. If it isn’t, then you have something wrong with the pump itself, or the pump stroke.

  • Make any adjustments to the pump stroke as set forth in the carburetor kit instructions.
  • Keep in mind any carburetor type will have multiple pump sizes used, unless you matched your carburetor number when ordering parts, you could have an incorrect pump. 
Updated on 04/21/2022

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