How to rebuild a Rochester Quadrajet 4MV carburetor
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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. | 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. | ||
| − | ====Airhorn location==== | + | Before removing the adjustment screw from the carb body (airhorn-adjusted APT), count the turns it takes to lightly bottom the adjustment screw and write it down before removing the screw- if it needs to be removed; often it can just be left in place. Use this setting as a baseline at reassembly. |
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| + | ====Airhorn adjustment 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. | ||
| − | ====Baseplate location==== | + | ====Baseplate adjustment location==== |
| − | File:Apt baseplate plug and wire.jpg | + | Earlier Q-jet carbs have a baseplate APT adjustment location. A hole centered between the idle mixture screws (plugged at the factory) has an adjustment screw that raises/lowers the power piston to set the APT adjustment. |
| − | File:Apt base plate.jpg | + | |
| + | These baseplate-located APT adjustment screws and wire assemblies are subject to being heavily carboned up and stuck in place, If the carb has been in service long, the baseplate needs to be removed and the APT assembly removed and thoroughly cleaned before being adjusted. Be sure to measure how far from the baseplate the wire spring protrudes BEFORE removing it so it can be returned to the same position as a baseline setting. | ||
| + | {| | ||
| + | |[[File:Apt baseplate plug and wire.jpg|thumb|center|400px|Red arrow points to baseplate APT plug location; yellow arrow shows wire that lifts the power piston so set the APT adjustment]] | ||
| + | |[[File:Apt base plate.jpg|thumb|center|700px|A = APT adjustment screw; B= wire spring that lifts/lowers power piston as screw is adjusted (yellow arrow at left); | ||
| + | C = power piston having bottom pin (pin not used with airhorn-adjusted APT)]] | ||
| + | |} | ||
====Aneroid or spool location==== | ====Aneroid or spool location==== | ||
| − | + | There were also additional Q-jet designs used mainly in 1975 only, that used an auxiliary enrichment system in addition to (or in some cases instead of) the primary PP system. One has an accordion-shaped aneroid "bellows" that lifted/lowered a single metering rod in a jet; it used no vacuum or power piston to operate it. The aneroid bellows changed the air/fuel ratio in accordance with the barometric pressure. Another was identical to the aneroid except the aneroid was deleted and was replaced by a filler spool that did not respond to barometric pressure; instead it was adjusted to give the correct air/fuel ratio according to ambient conditions only. These systems was found to be lacking, so were discontinued. | |
| − | The | + | The other system uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one fixed jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system. |
{| | {| | ||
|[[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: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]] | |[[File:Aneroid2.jpg|thumb|center|360px|Aneroid shown with airhorn removed]] | ||
|} | |} | ||
| + | [[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|left|600px|Early APT, ca. 1975]] <br style="clear:both"/> | ||
===Quadrajet tuning and repair parts=== | ===Quadrajet tuning and repair parts=== | ||
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