SupremeDeluxe

Having a stumble when attempting a brisk part-throttle acceleration with my tripower car. It did not have this issue last year that I recall. Just came up when I took it out of storage a few days ago.

Have checked all the usual items. Only unusual thing I am seeing is that the accelerator pump is delivering a shot only with very sudden throttle movement. AP is adjusted properly (0.015" margin at WOT), red cam in #1 position (OEM setup for 3660). A half-hearted throttle movement (as in not a rapid "snap") is getting a partial or no-shot from the AP. I took the float plug out (engine off) and observed that on a slow throttle movement from closed to WOT, the fuel level in the bowl increases and no shot is being delivered through the nozzle.

Bad check ball/valve in the float bowl? How to fix? Other things to look at?

tbarb

It's probably going to be a process of elimination, there should only be approx .015 between the accelerator pump check ball (under the pump) and the little bar that holds it in place. This keeps from wasting the first part of the pump shot seating the ball.

I use a bread wire tie with the paper removed as they are about .013 and you can slide it in there easily enough. Slow throttle movement will not show a pump shot as the check ball under the pump will be off it's seat allowing fuel to go back into the float bowl.

There could also be leakage at the metering block gasket interface where it meets the main body. Think about changing the gasket and try the #2 position on the red cam if the car idles a little higher.

MelWff

the Holley pump is pretty simple as you know, could only be the check valve being stuck or the diaphragm, so remove the bowl and clean it and replace the diaphragm.

Faslane

diaphragm is probably torn, that to our good fuels these days.

SupremeDeluxe

On closer inspection, the problem has nothing to do with the accelerator pump. What seems to be happening is that we have a lean stumble when accelerating hard from cruise. As if the secondary carbs are trying to open too quickly. I removed the secondary diaphragm housing from the front carb and found a large piece of casting flash on the mating surface where the secondary vacuum port gasket resides. I think it was causing a vacuum leak under high venturi vacuum when the primary blades go wide. I cleaned everything up with an india stone and need to find the cork gasket before I put it together. Amazing to me that this wasn't causing a problem before. Carbs are new Holley repros that I put on the car 2 years ago. What is even more amazing is how bad Holley's quality is. These came with so many defects out of the box, that they almost need to be rebuilt and blueprinted before being put on a car.

SupremeDeluxe

To close the loop---my original diagnosis was correct. The accelerator pump check ball was intermittently not sealing on it's seat, causing a weak or incomplete pump shot. I had to take the bowl and submerge it in a small container of mineral spirits while actuating the pump to understand what was happening.

I took a countersink tool by hand and cut a perfect edge on the check ball seat. Put the ball in and peened it down on the edge gently with a hammer. Reinstalled the retaining bail, and it works perfectly now. Runs like a bear.

Holley's machining and overall QC on these carbs is just terrible. I will never buy a new carb from them again. Don't take for granted that anything new out of the box from them will work properly.