Editing How to rebuild a Rochester Quadrajet 4MV carburetor (section)
Jump to:
navigation
,
search
====APT adjustment==== 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. The following is with the carb disassembled: 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. 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. 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. [[File:Apt airhornhole plug.jpg|thumb|400px|left|Q-jet showing APT (yellow arrow) and plug removed (red 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 with the carb on the engine. <br style="clear:both"/> Now, you can use the choke or your hand hand to partially block some of the air entering 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 mixture screws after each APT adjustment as well. Another method (from SMI, IIRC): "Simply turn the screw DOWN (CW) 1/2 turn at a time until you experience a "lean surge". A lean surge will feel like someone is moving the throttle, or you get a hesitation on light throttle acceleration). Then turn the screw UP (CCW), until the lean surge goes away. If you are experiencing an "off-idle stumble", you can turn the screw up 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.
|
Editing help
(opens in new window)
Personal tools
Log in / create account
Namespaces
Page
Discussion
Variants
Views
Read
Edit
View history
Actions
Search
Navigation
Main Page
Recent changes
Random page
Help
All articles
Start a new article
Hotrodders forum
Categories
Best articles
Body and exterior
Brakes
Cooling
Electrical
Engine
Fasteners
Frame
Garage and shop
General hotrodding
Identification and decoding
Interior
Rearend
Safety
Steering
Suspension
Tires
Tools
Transmission
Troubleshooting
Wheels
Toolbox
What links here
Related changes
Special pages
Terms of Use
Copyright
Privacy Policy
Your Privacy Choices
Manage Consent