How to rebuild a Rochester Quadrajet 4MV carburetor
m (→Adjustable part throttle (APT) setting) |
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Another seldom-mentioned adjustment that can be made to a Q-Jet, is the travel limiter for the PP. It can be adjusted (screwed) up or down to allow the primary needles to sit higher or lower at max vacuum, and is another tuning aid when dealing w/long duration or overlap cams. This adjustment is referred to as the APT or "adjustable part throttle" setting. | Another seldom-mentioned adjustment that can be made to a Q-Jet, is the travel limiter for the PP. It can be adjusted (screwed) up or down to allow the primary needles to sit higher or lower at max vacuum, and is another tuning aid when dealing w/long duration or overlap cams. This adjustment is referred to as the APT or "adjustable part throttle" setting. | ||
| + | There are two common types of APT adjustment locations: later carbs use a plugged hole in the airhorn, earlier carbs use a plugged hole in the baseplate. | ||
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| + | ====Airhorn location==== | ||
It is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located. | It is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located. | ||
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If you are experiencing an "off-idle stumble", you can turn the screw up at 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble. | If you are experiencing an "off-idle stumble", you can turn the screw up at 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble. | ||
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| + | ====Baseplate location==== | ||
| + | File:Apt baseplate plug and wire.jpg | ||
| + | File:Apt base plate.jpg | ||
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| + | ====Aneroid or spool location==== | ||
| + | A third type of APT adjustment was used briefly during the 1975 model year, mostly. It used an altitude compensating aneroid that changed the air/fuel ratio in accordance with the barometric pressure. It consisted of a sealed bellows that was accordion-shaped that lifted/lowered a single metering rod in a jet. This system was found to be lacking, so it was discontinued. Carbs using it can be converted back to normal use by plugging the jet and removing the aneroid assemble, but the carb will be lacking any APT adjustment. | ||
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| + | The spool was used in place of the aneroid but was not altitude compensating, but was otherwise basically the same. | ||
| + | {| | ||
| + | |[[File:Aneroid.jpg|thumb|center|350px|Red arrow points to aneroid adjustment screw through hole in airhorn. Hole is plugged with a cup plug.]] | ||
| + | |[[File:Aneroid2.jpg|thumb|center|360px|Aneroid shown with airhorn removed]] | ||
| + | |} | ||
===Quadrajet tuning and repair parts=== | ===Quadrajet tuning and repair parts=== | ||
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