This question was recently asked by one of our customers.
1967 GTX, 4127s have been into this card 50 times. Can not get why the accel’ pump won’t work until half throttle. It seems it’s in the body. I have three 4127s, the other two work. All adjustments are spot on, bench test seems good, put on car, no dice. Traded inlet check, plunger, top, needle, squirt-er, cleaned bore, blows clear, take out squirt-er, hold finger on hole, no leak back to bowl………….I have only one hair left to pull, HELP
like a bad accelerator pump.
Assuming the distributor is OK, turn your attention to the accelerator pump circuit. You 1st want to be sure the adjustment to the pump arm is such that when you press on the throttle, the pump starts to move right away. Any slack will cause it to hesitate.
The next thing is to see if any fluid is escaping when the pump is depressed. You will need to remove the top of the carburetor (should be off the car), and remove the pump jet housing (I would call this the pump discharge). Fill the pump housing with fluid. Plug the discharge with your finger and press down on the accelerator pump. You should get some pressure. If not, then the fluid is leaking back through the check ball into the float bowl.
Assuming there is nothing leaking back, fill the pump housing with fluid, and press down on the pump. Fluid should shoot up through the discharge hole right away. If not, then there is a blockage somewhere.
By the way, be sure the check weight is in the discharge hole.
We use mineral spirits for this type of testing.
Make sure your secondary air valves are working. When the secondary plates are opened, the air valves will not allow air through the plates until the engine can burn the extra air and fuel mixture. This prevent hesitation during acceleration.
Another possibility is the metering rod springs. These springs move the metering rods depending on the engine vacuum and bad or incorrect springs could also cause hesitation. This is especially true when running any kind of performance cam resulting in lower vacuum. Eperiment with different springs to see if the hesitation is removed.