Accelerator Pump Stroke Adjustment
The accelerator pump stroke has 3 possible adjustments depending on the ambient temperature.
The center hole is for normal operations and by default you should use this hole.
Hole 1 will decrease the fuel quantity – Use for hot weather.Hole 3 will increase the fuel quantity – Use for cold weather.
To change the hole:
- Remove the pump rod to pump arm retainer.
- Remove the rod from the arm.
- Remove the pivot pin retainer which can’t be seen in this photo because it in the back of the lever.
- Move the pivot pin to the hole desired.
- Install the pivot pin retainer.
- Position the pump rod end into the pump arm and install the retainer.
As you step on the throttle, the carburetor needs a temporary shot of extra fuel while it is transitioning to power. This is accomplished by the accelerator pump circuit.
When the accelerator pump returns to the top position, fuel enters the pump well from the float bowl via the intake check valve. With the partial vacuum from the pump moving up and the weight of the fuel, the check ball lifts up to allow the fuel in. When the pump is pushed down the check ball seats in the hole not allowing the fuel to return to the float bowl.
As the accelerator pump pushes down, the fuel is pushed through the discharge channel and out the discharge hole. The pressure lifts the discharge needle and the fuel passes through. When the accelerator pump is not being used, or the pump well is filling up, the discharge needle is seated. This keeps fuel from siphoning off into the carburetor bore, which it would do due to the vacuum created in the carburetor bore.
Hesitation can be caused by ignition, but it can also be caused by a problem in the accelerator pump circuit.
You can do the quick accelerator pump test as found here, or you can test the pump while on the bench. This test is done with the top removed.
- With the accelerator pump pushed all the way down.
- Fill the float bowl with mineral spirits for testing.
- Pull the pump up. The fluid from the float bowl should fill the pump well.
- Hold the discharge needle down with a brass drift punch and push the accelerator pump down. You should feel some pressure against the pump. This will indicate if the discharge needle is sealing. If not tap lightly on the discharge needle to seat it.
- Do the test again, but this time let the discharge needle move on it’s own. Fluid should flow out of the discharge.
The channels going to the pump well and also the discharge are very small, so be sure they are clear and clean.