Quadrajet question
09-18-2002, 09:37 PM
#1
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Quadrajet question
Primary jet question:
Will too large of a primary jet affect idle quality?
Airscrews do not seem to clean it up - dirty transom,
sputters in gear at low rpm like it's loading up.
Will too large of a primary jet affect idle quality?
Airscrews do not seem to clean it up - dirty transom,
sputters in gear at low rpm like it's loading up.
Last edited by BGIII; 09-18-2002 at 09:40 PM.
09-18-2002, 10:28 PM
#2
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Posts: n/a
Quadrajet
The only time the engine pulls fuel through the primary jets is when the engines throttle butterflys are open a considerable amount (1/4 to 3/8 throttle). The Idle and Transition circuit is feed by a separate air/fuel circuit.
The usual reason why you would have no idle mixture screw adjustment is because the throttle butterflys are open to far (the throttle idle screw is turned in to far to keep the engine idling). Look at the bottom of the barrel just above the closed throttle blade and you will see some slots (called transition slots). The bottom of the slot should barely be exposed below the blade at an idle.
If you can't close the throttle blades without the engine killing then you can do one of two things.
The first is to increase idle speed by adding some ignition timing at idle. Try 10 or 12 degrees instead of 8. This should allow you to close the throttle blades and pick up some mixture screw adjustment.
The other is to add more air to the intake manifold through a separate circuit around the throttle plates. Some Quadrajets actually have a small hole that bypasses the barrels.
Most guys with Holleys drill a small hole in the butterfly to let more air pass through (inabling them to close the butterflys down and close off the transition circuit).
Of course the biggest problem with Holleys is a blown powervalve diaphragm. This will allow fuel to pass directly from the fuel bowl into the intake manifold. But this results in an extremely rich condition (doesn't sound like your problem).
Another problem with Holleys is the metering block has an internal idle metering circuit that is not easily accessed. Soaking the block in carb cleaner may unblock the obstruction but usually not. The only solution is a new metering block or carburetor. This is the reason why sometimes a Holley carb rebuild doesn't always work (and the reason why there are so many Holleys sitting around not being used).
Sincerely
Dennis Moore
FAMILY AND PEWRFORMANCE BOATING MAGAZINE
PS The Edelbrock/AFB/Weber/Merc carb is great for us weekend carb rebuilders because all of the metering circuits are easily inspected and cleaned when the carb is disassembled. Usually the Quadrajet can be rebuilt as well.
The usual reason why you would have no idle mixture screw adjustment is because the throttle butterflys are open to far (the throttle idle screw is turned in to far to keep the engine idling). Look at the bottom of the barrel just above the closed throttle blade and you will see some slots (called transition slots). The bottom of the slot should barely be exposed below the blade at an idle.
If you can't close the throttle blades without the engine killing then you can do one of two things.
The first is to increase idle speed by adding some ignition timing at idle. Try 10 or 12 degrees instead of 8. This should allow you to close the throttle blades and pick up some mixture screw adjustment.
The other is to add more air to the intake manifold through a separate circuit around the throttle plates. Some Quadrajets actually have a small hole that bypasses the barrels.
Most guys with Holleys drill a small hole in the butterfly to let more air pass through (inabling them to close the butterflys down and close off the transition circuit).
Of course the biggest problem with Holleys is a blown powervalve diaphragm. This will allow fuel to pass directly from the fuel bowl into the intake manifold. But this results in an extremely rich condition (doesn't sound like your problem).
Another problem with Holleys is the metering block has an internal idle metering circuit that is not easily accessed. Soaking the block in carb cleaner may unblock the obstruction but usually not. The only solution is a new metering block or carburetor. This is the reason why sometimes a Holley carb rebuild doesn't always work (and the reason why there are so many Holleys sitting around not being used).
Sincerely
Dennis Moore
FAMILY AND PEWRFORMANCE BOATING MAGAZINE
PS The Edelbrock/AFB/Weber/Merc carb is great for us weekend carb rebuilders because all of the metering circuits are easily inspected and cleaned when the carb is disassembled. Usually the Quadrajet can be rebuilt as well.
