Q-Jet Problem: No Accel Pump Shot
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Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Q-Jet Problem: No Accel Pump Shot
It seems no matter how many Q-Jets I build over the years, there is always a new problem to be discovered. This time, I received a ’69 Vette Q-Jet with a customer complaint, “well plugs leaking – hard to cold start – no fuel in bowl.”
A leak check showed that the bowl plugs were just fine (the leaky plug thing is grossly over-rated), and the carb held fuel over night. But the accel pump did not produce a pump shot to enable the engine to start.
Disassembling the carb showed that a non-ethanol-compatible pump has been installed, and the pump was seized in the bore. So a good accel pump was installed and tested without the airhorn installed. The pump did not work, and could not be pushed down into the pump bore with the bore filled with fuel: the pump would simply compress against its spring.
The pump checkball pin and the ball were removed, and the passages were blown out and verified to be open. The pump would operate in the pump bore with the ball and pin removed. Install the ball and pin, and the pump would lock up in the pump bore…
I loosened the ball pin up ½ turn, and suddenly the accel pump circuit operated perfectly. Removing the pin and comparing it to a “real” Q-Jet pin showed that the aftermarket replacement ball retention pin that was installed in the carb was .040” too long: The pin should have an overall length of .630”, and the pin in this carb measured .670”. This additional length made the pin push the checkball firmly down into the seat, effectively blocking the entire accelerator pump fuel flow circuit.
Installation of a correct pin solved the entire problem, and the carb is now cold-starting and running perfectly.
Photo showing the correct pin (left) versus the long aftermarket pin on the right:
These aftermarket, incorrect parts are very common in commercially rebuilt carbs, but these cheap repro parts are making their way into other carbs as well during rebuilding...
Lars
A leak check showed that the bowl plugs were just fine (the leaky plug thing is grossly over-rated), and the carb held fuel over night. But the accel pump did not produce a pump shot to enable the engine to start.
Disassembling the carb showed that a non-ethanol-compatible pump has been installed, and the pump was seized in the bore. So a good accel pump was installed and tested without the airhorn installed. The pump did not work, and could not be pushed down into the pump bore with the bore filled with fuel: the pump would simply compress against its spring.
The pump checkball pin and the ball were removed, and the passages were blown out and verified to be open. The pump would operate in the pump bore with the ball and pin removed. Install the ball and pin, and the pump would lock up in the pump bore…
I loosened the ball pin up ½ turn, and suddenly the accel pump circuit operated perfectly. Removing the pin and comparing it to a “real” Q-Jet pin showed that the aftermarket replacement ball retention pin that was installed in the carb was .040” too long: The pin should have an overall length of .630”, and the pin in this carb measured .670”. This additional length made the pin push the checkball firmly down into the seat, effectively blocking the entire accelerator pump fuel flow circuit.
Installation of a correct pin solved the entire problem, and the carb is now cold-starting and running perfectly.
Photo showing the correct pin (left) versus the long aftermarket pin on the right:
These aftermarket, incorrect parts are very common in commercially rebuilt carbs, but these cheap repro parts are making their way into other carbs as well during rebuilding...
Lars
Last edited by lars; Jun 18, 2014 at 10:49 AM.
Thread Starter
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
...and there are more and more of these "tiny" problems with these carbs as the years go by, making it very difficult for the average enthusiast to identify problems, correct the issues, and get things running right. The days of tearing a carb down, cleaning it up, putting a "kit" in it and re-assembling it are long gone: Welcome to the age of detective work, problem identificaton, and parts sourcing to find replacements for incorrect parts, bad parts, and missing parts.
