When I’m rebuilding my carburetors, I will usually test the accelerator pump circuit before closing up the float bowl. The Holley 94 carburetor uses a check ball in the accelerator pump well and I want to make sure it doesn’t need seating.
1st, you need to know why the check ball is there. The accelerator pump well has two holes. One is used for the fuel intake (from the float bowl) and the other is for sending fuel to the main discharge and into the carburetor venturi. When the accelerator pump is on the up stroke (foot off the gas), fuel is pushed into the well via the check ball hole. The weight of the fuel entering the well will lift the check ball off of the hole, allowing fuel into the accelerator pump well. Now when you press on the gas pedal, we want the fuel to pump out to the main discharge and not back to where it came from, which would cause a hesitation from lack of fuel. That is the job of the check ball. It closes off the inlet hole keeping the fuel from back pedaling. When the check ball doesn’t seat into the hole completely fuel may bypass the check ball. We will test for that problem as well as some other things.
I use mineral spirits when I test because it doesn’t deteriorate rubber like gas and ethanol does, so the carburetor can sit on the shelf for awhile if I need it to. It also doesn’t stink up my shop like gas does. At this point the main jets are installed and the drain plugs are installed, nothing else. Using a turkey baster, I add enough to the float bowl to fill it about 1/4 full and I squirt some in the accelerator pump well.
For the 1st test I force the accelerator pump down the well and I watch to make sure some fluid comes out of the main discharge. That eliminates the possibility of the tube to the discharge being plugged.
The next thing I do is put my finger over the main discharge hole and again I force the accelerator pump down the well. This time I want to make sure I get a little pressure against the pump and that the fluid doesn’t escape past the check ball and back into the float bowl. If it does, I seat the check ball by tapping on a brass drift punch sitting on top of the check ball.
For the last test, I insert the check weight into the main discharge and using a brass drift punch, I hold the weight down. I again force the accelerator pump down the well to make sure fluid doesn’t come out of the discharge hole. The check weight should shut the hole off. The same tapping using a brass punch can be used here also.
If all of these tests pass, I can feel good knowing that I’m not going to have any accelerator pump problems once the carburetor is out the door.
This same procedure is illustrated in this video.