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Old 05-24-2012, 10:40 PM
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rochester 2 barrel m2mc

i have rebuilt a carb on 79 3.8 firebird someone before me has adjust the metering rob screw does anyone know how many turns it should be

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Old 05-25-2012, 01:58 AM
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Originally Posted by mrdjsweet03
i have rebuilt a carb on 79 3.8 firebird someone before me has adjust the metering rob screw does anyone know how many turns it should be
The adjustment you're talking about 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 all be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel you have, 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.

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 I always do for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting w/o a gasket in place. The carb may be richer than needed in that position (or not, depending), but better than than too lean. But in that position the engine can be run safely and further fine tuning can be done, and w/the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted.

After setting the adjuster even w/the body casting, tap out the aluminum plug in the air horn casting using a punch and ball peen hammer, etc.(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.

From another thread on this:

I use the choke or my hand to partially block some of the air into the carb. I set the APT to where there's a slight increase in RPM as I "choke" the carb some, while running @ 2000 RPM. Not so much that it stumbles or tries to stall, just enough to see if- and how- the RPM changes. This indicates I'm just at the lean edge.

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.

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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