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Communicating with Your Computer Controlled Carb in Your Corvette

Technically Speaking - Communicating with Your Carburetor: Part 1

James Berry Jul 21, 2017
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Q Hello. I have gotten myself into a bit of trouble with my computer controlled Quadrajet. My carburetor is running very rich, with black smoke coming out of the tailpipe. I removed the screw in the top of the air horn to replace the O-ring but there was nothing wrong with the O-ring.

I have determined that the mixture control solenoid has failed; it is not clicking with the key in the Run position.

I have a lot of experience rebuilding Quadrajet carburetors and I have no problem removing the top of the carburetor and replacing the mixture control solenoid. The problem is I have never set the metering rod travel.

I would also like the replace the throttle position sensor, when I sweep the voltage about mid-throttle there is a portion where the voltage drops to “0” then it jumps back up to 3 volts.

Could you explain to me how the metering rods work and how to adjust the metering rods? Is the throttle position sensor serviceable? If so, could you explain how to replace it? Also, is there a throttle position sensor voltage adjustment?

Thanks,
Luke

A Luke, we are going to cover your question in two articles. This month we are going to concentrate on replacing the mixture control solenoid and how to set the rich stop and the lean stop.

In the 1981 model year, General Motors introduced an electronic carburetor from the Rochester Products Division in two- and four-barrel versions. These carburetors are termed “Feedback Carburetors” and are easy to identify by the two electrical wires going into the float bowl of the carburetor.

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These carburetors have the ability to self-adjust the air/fuel ratio by the use of a mixture control solenoid housed inside of the main bowl of the carburetor.

Basically, this is how it works. Using the Computer Command Control (CCC) system an oxygen sensor monitors for a rich or lean air/fuel mixture in the exhaust system. This value is sent to the engine controller and, along with the other sensors, determines what adjustment should be made to try and maintain the perfect stoichiometric air/fuel ratio of 14.7:1.

Remember, it is common for these oxygen sensors to become worn over time and not read accurately due to the age of the sensor. The sensors are wear items and should be changed periodically.

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How the Fuel Metering is Controlled on Conventional Carburetor Compared to a Feedback Carburetor

On a conventional carburetor the metering rods are spring-loaded down into the main jets causing the air/fuel mixture to be lean. As engine load increases a vacuum is produced, overriding the spring pressure and allowing the metering rods to come out of the jets and enrich the air/fuel mixture.

The feedback carburetor uses the mixture control solenoid to adjust the air/fuel mixture. The mixture control solenoid uses two wires, one a 12-volt constant key on feed and the other a ground wire that is cycled by the engine controller to adjust the air/fuel mixture.

When the solenoid is energized, a magnetic field is induced and the plunger that sits over the two metering rods is pulled down into the solenoid and forces the mixture lean.

When the solenoid is de-energized, the spring pressure pushes the metering rods up and out of the main jets, which forces the mixture rich.

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How to Adjust the Metering Rods on the Computer Command Controlled Carburetor

There are two metering rod adjustments on the Computer Command Control carburetor, they are called the rich stop and the lean stop. There are a few special tools required for this adjustment.

A. Positioned in the top of the carburetor, or air horn, is the rich stop. It defines how far the metering rods can travel out of the main jets when the mixture is driven rich.

B. Located in the main body of the carburetor is the lean stop. It defines how far down into the main jets the metering rods can travel. This reduces fuel flow and drives the mixture lean.

When the ignition is in the On position or the engine is running, a rapid clicking sound emitting from the carburetor is the metering rods cycling in and out of the main jets.

The metering rods cycle at 10 hertz, or cycles per second. The amount of time the mixture control solenoid stay energized or degenderized determines how long the metering rods stay in or out the main jets, causing the air/fuel mixture to be driven rich or lean.

This can be observed and adjusted by monitoring the dwell for the mixture control solenoid. The minimum dwell is 0 degrees and the maximum dwell is 60 degrees, so 30 degrees would be perfect.

A. If the dwell reading is on the high side (over 30 degrees) the metering rods are spending more time in the main jets, driving the air/fuel mixture lean.

B. If the dwell reading is on the low side (below 30 degrees) the metering rods are spending more time out of the main jets, driving the air/fuel mixture rich.

C. If the metering rods are spending an equal amount of time in and out of the main jets the dwell will read 30 degrees. So try and set the dwell as close to 30 degrees as possible while the engine is warmed up and running. It is normal for this reading to vary a small amount.

You can monitor and set the dwell reading using a scan tool connected to the underdash diagnostic connector Assembly Line Data Link (ALDL). If you do not have the use of a scan tool you can use a standard dwell meter.

If using a standard dwell meter, always set it on the six-cylinder (60-degree) scale (no matter if the vehicle is equipped with a six-cylinder or eight-cylinder engine). The primary lead of the dwell meter should be hooked to the hood mixture control green connector located near the carburetor.

Luke, this month’s segment is only part of the puzzle. Next month we will cover the air bleed screw in the top of the air horn, removing the top of the carburetor, replacing and adjusting the throttle position sensor (TPS), adjusting the air/fuel mixture and removing the plugs to gain access to all of the carburetor adjustments, including the rich and lean stop.

Photography by James Berry

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