2 Jet Accelerator Pump Circuit & Testing

2 Jet Hesitation

When you step on the throttle, the engine seems to bog down and in some cases die.

A bad distributor advance can cause a hesitation when stepping on the gas, but we aren't addressing that here.

Stepping on the throttle is where the accelerator pump circuit comes into play. When the pump moves up (let up on throttle), the accelerator pump well is filled with fuel. Some get the fuel by filling over the top of the well through a slot on the side, others use a intake at the bottom of the well which is shut off with a check ball.

In most 2 jets the small aluminum check ball goes in the pump well. In a few 2 jets the large aluminum check ball goes in the pump well. At any rate aluminum is used because it is light and easy for the fuel to lift up.

With the engine off, pump the throttle while looking down the carburetor throat. You should see 2 strong squirts coming out of the venturi. If not, then you have a blockage somewhere.

The small holes in the venturi can be blocked with ethanol residue. run thin wire down the holes.

Check the gasket under the venturi, they don't always fit well and have to be trimmed. If they are too big then the venturi won't site flat.

Put fluid in the pump well and press the accelerator pump down. Fluid should come out of the main discharge.

The main discharge should have a check ball, spring, then a T to hold it all in. This is where the bigger stainless steel check ball resides.

Testing the accelerator pump circuit on a Rochester 2 Jet carburetor.

This article only refers to the type of 2 jet that uses a check ball in the bottom of the pump well. Not all carburetors do.

When rebuilding your 2 jet you may want to test the accelerator pump circuit before closing up the carburetor. Here are the steps you need to take.

Place the smaller and aluminum (when provided), check ball into the bottom of the accelerator pump well. Not all 2 jets will have this check ball and a few will use a bigger aluminum check ball. Fill the float bowl and the pump well with mineral spirits.

Put the pump in the well and push it down toward the bottom of the well. The check ball will seal the hole, keeping the fluid from going back into the float bowl and the fluid will be forced through the 2nd hole through the main discharge. You should see fluid come out of the main discharge.

The main discharge should have the bigger stainless steel check ball, then the gold colored spring (don't worry if it is bent), then the T. Stake the T so that it doesn't pop out.

Now test to see if the check ball seals by holding your finger over the main discharge hole and pressing down on the pump again. You should feel a slight pressure on the accelerator pump and there should not be any bubbles or some other sign that would indicate the check ball is leaking and fluid is being forced backwards.

The larger check ball in the main discharge tube must also be tested. Hold the check ball down with a brass drift punch and push the accelerator pump down. You might get but a slight amount of fluid past the check ball, but you should also feel a slight pressure.

Replace the venturi cluster and test again by pushing the pump down. You should get a strong squirt out of both sides of the cluster. If not run thin wire through the small passages. They may be plugged with ethanol residue and cleaners will not remove the residue.

To re seat a check ball, using a brass drift punch, hold the check ball down into the hole with the drift punch and tap a few times with a hammer. Don't get carried away, or you will get the check ball stuck. The hole in the carburetor you are trying to form is soft metal and doesn't take much to change.

Now you can be confident your accelerator pump circuit is doing what it should.

Is the check ball stuck in either hole? Turn the carburetor over and while heating the bottom where the check ball resides tap the carburetor bowl on the bench. It will most likely fall out when hot enough.