How to rebuild a Rochester Quadrajet 4MV carburetor
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GM/Delco PP springs come in packs of 10 springs unless you can talk your way into buying less. | GM/Delco PP springs come in packs of 10 springs unless you can talk your way into buying less. | ||
| + | ===Adjustable part throttle (APT) 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. | 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. | ||
| − | It is shown | + | 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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| + | [[File:Apt screw details.jpg]] <br style="clear:both"/> | ||
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| + | This adjustment is called the "adjustable part throttle" setting, or just "APT" for short. Using those terms will help you find additional info on the subject should you want it. The setting procedures will be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel as well, which is the primary side of a Q-jet carb, anyway. | ||
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| + | The following is with the carb disassembled: | ||
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| + | If you don't have the tool that fits the adjuster screw, carefully slot the screw yourself so you can use a small straight blade screwdriver to adjust it. | ||
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| + | You may read that 2 to as much as 3-1/2 turns CCW from lightly bottoming the adjustment screw is a starting point. What may also be done for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting without a gasket in place. The carb [I]may[/I] be richer than needed in that position (or not, depending. But in that position the engine can be run safely and further fine tuning can be done, and with the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted. | ||
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| + | [[File:Apt airhornhole plug.jpg|thumb|400px|left|Q-jet showing APT (red arrow) and plug removed (yellow arrow)]]After setting the adjuster even with the body casting, remove the aluminum plug in the air horn casting using a punch and ball peen hammer (''carefully''- support the casting so when you tap out the plug there's no undue pressure put on the casting), then tap the hole for a screw-in plug. That way you can easily adjust the power piston w/the carb on the engine. | ||
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| + | Now, you can use the choke or your hand hand to partially block some of the air into the carb. Set the APT to where there is a slight increase in RPM as the carb is partially choked, while running at about 2000 RPM. Don't choke it so much that it stumbles or tries to stall, just enough to see if- and how- the RPM changes. The slight increase in rpm indicates the ATP is set just at the edge of being lean. Further adjustments can be made as needed, be sure the engine is always fully warmed up and that the idle mixture screws are spot on before adjusting the APT, and readjust the idle mixtrure screws after each APT adjustment as well. | ||
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| + | Another method (from SMI, IIRC): "simply turn the screw DOWN (CW) 1/2 turn at a time until you experience a "lean surge". (It will feel like someone is moving the throttle, when their not, or a hesitation on light throttle acceleration). Then turn the screw UP, until the lean surge goes away. | ||
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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. | ||
===Quadrajet tuning and repair parts=== | ===Quadrajet tuning and repair parts=== | ||
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