How to test the accelerator pump while on the bench.

Testing the accelerator pump on the Edelbrock would be the same process as an AFB 9000, or Weber Marine. This test is best done with the carburetor off of the car. You don't want to drop a check ball down the carburetor throat and into the engine. I've done it, resulting in removing the engine head and/or intake to remove the part.


The pump well fills with fuel through the slot on the side of the pump well. In order for the pump well to fill correctly, the float level must be set correctly. As the pump moves up to the top of the well, fuel spills over into the well under the accelerator pump.


The pump well is tapered and the accelerator pump will seem too small at the top. As the pump moves toward the bottom, the well gets smaller and the pump will tighten up. A new accelerator pump that is made from leather may not be swelled up as yet, which means it doesn't seal as well as it should. Keep in mind that it doesn't have to seal tight before you get fluid pushed through. It shouldn't take much to pump the fluid through the passage.


To test:

Fill the bowl with fluid. Use something besides gasoline, which of course is explosive. We use mineral spirits (odorless) for all of our testing.


Insert the accelerator pump and push it to the bottom. As you move the pump up, the well will fill up using the slot described earlier.


Push the pump down and watch it squirt gas out of the main discharge. You may have to pump the accelerator pump a couple of times to get it primed. Also it may not squirt until the pump is part way down the well. The well is tapered and also the pump cup may not be swelled up yet.


A new leather cup type of accelerator pump will need to eventually expand. Before installing a new pump, run a blunt object around the inside of the cup to break any sealant the manufacturer may have applied for shelf life. Apply a couple drops of oil to the pump. It will eventually expand to fit. Please note, it may never fit tight and isn't supposed to. Avoid soaking the pump in oil overnight. This will make the leather too spongy and will probably fail.


For rubber cup pumps, apply a dab of lithium grease to the cup. This will help lubricate the well. Today's gas has very little lubricant causing premature pump failure. Polishing the pump well with crocus cloth will also help. The pump needs a smooth gliding surface.


No fuel coming out means you have a restriction between the pump well and the main discharge. Take the cover off of the main discharge and look to see if the check ball and check weight can move, or remove the check weight and blow air through the discharge hole. The air should go through to the pump well. There may be a restriction in the passage. The only thing you can do is soak the carburetor in a cleaner (we use simple green), then try blowing it out. Also don't discount the discharge cover. The small holes may be plugged.


Next test is to see if the discharge check is working. There may be a check ball with a small check weight above it. Some AFB's, or Weber Marine use a check ball and a spring. Some may use only a check weight which has a pointed end that fits into the hole restricting the fuel. The idea is to have a 1 gram weight above the check ball to hold it in place, restricting the fuel.


Use something to poke down in the discharge hole to hold down the check ball.


Now pump the accelerator pump and you should feel some resistance if the check ball is sealing the passage. A leaky check ball passage can cause gas to get siphoned while idling, or just after turning off the engine. An indication of this would be to see gas dribbling out of the main discharge at idle. That should not happen.


Gently tap on the check ball so that the trough forms to the check ball. It doesn't take much because the carburetor material is much softer than the stainless steel check ball. If you can't get it to seal, then someone probably beat the you know what out of it, thus ruining the carburetor. There is no way to fix it at that point.


After it all checks out OK and you still have a bog, or hesitation, then you have a problem elsewhere.


The distributor advance may not be working correctly. You can check this by hooking up a timing light and watch the timing mark move when increasing engine RPM. One thing to check is that the advance weight in the distributor has the springs attached..


One other possibility is the idle discharge. Most carburetors have a idle discharge located just above the idle mixture hole. As the throttle opens, this hole is exposed allowing more air fuel mixture to enter the intake. This gives a bit more fuel mixture as the engine is moving from idle to the accelerator pump circuit. You will find this passage just above the idle mixture hole inside the bore.