Reply to "Stumble!!"

The clearance given by Holley between the accellerator/pump diaphram is necessary because 1)the clearance changes as the engine gets hot 2)the clearance changes when there is fuel in the pump assembly.

If you are adjusting out all clearance on the bench, that is wrong. You are limiting the amount of designed travel of the pump diaphragm. You are also reducing the amount of fuel held in the diaphragm assembly. Then you won't be getting enough shot to cover a sudden throttle opening and the engine will bog or backfire from a lean condition.

Holley didn't imagine that clearance number.


I believe that the white one is the original pump actuator.


The stumble can also be caused by too much fuel in the pump nozel.

The nozel that you want should be marked either .024 or .025. It should not have any tube extensions on it.

The tube extensions are for special application and will mis-direct the pump shot from where it is supposed to splash on the venturis.


You need a vaccum gauge to test this before you do anything.

You need to measure the engine vacuum at idle to verify that there are no engine vacuum leaks.

A blown power valve will show at this point with the engine showing anywhere from about 7 to 12 inches of vacuum at idle where it would normally be 15-17.

If you have low vacuum at idle and there are no external vacuum leaks then the power valve is blown.

You need not have a backfire to cause it to fail. It can dry out from having had fuel in it, then sat for enough time to let the fuel evaporate from the bowls.

With ethanol in the fuel like here in the US, that will cause a corrosion effect on the power valve materials and it can, and eventually will, cause the material to crack.

This is where you need to test the power valve. You need the special tool for that.

You will have two types of failures. One will be completely blown out, caused probably by a backfire, the other will show that the power valve has a slow leak in it and starts at 15 inches or so and within about 3 seconds will be at 0.

That is the corrosion failure.