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Communicating with Your Corvette Computer Controlled Carb: Part 4

Adjusting the Computer Controlled Feedback Carburetor

James Berry Oct 27, 2017
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Welcome back, in this installment we’re going to finish this series on the adjusting the computer controlled feedback carburetor. We will cover the air bleed valve adjustment, mixture screw tamper-resistant plug removal, mixture screw setting and troubleshooting the dwell.

Air Bleed Valve Adjustment
One of the most important things to remember is that the idle air bleed has a height setting that is set in relation to the travel of the metering rods. The rich stop and the lean stop metering rod travel must be set before the idle air bleed is adjusted.

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Idle Air Bleed Screw Access


The idle air bleed screw is located in the top of the carburetor air horn. To gain access to the idle air bleed screw you will need to drill out the two rivets to remove the idle air bleed screw tamper-resistant covering. If you remove the rivets while the carburetor is still on the vehicle, take care to make sure no fragments fall into the engine.

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Check for Idle Air-Bleed Vacuum Leak


A common problem is for the idle air bleed screw to leak vacuum at one of its two “O-rings. To check for this problem with the vehicle running and at operating temperatures simply cover all of the access to the idle air bleed screw with your fingers. If you notice a change in engine rpm suspect a leaking O-ring.

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Air Bleed O-Ring. The O-ring was so brittle that it fell apart during removal.


To inspect the O-rings for damage with the vehicle not running, gently turn the idle air bleed screw clockwise until it stops and record how far you turned it. Now turn the idle air bleed screw counterclockwise and remove it to inspect the O-rings for damage. After replacement simply reinstall the idle air bleed screw by gently tightening it until it bottoms out and then back the screw out the same number of turns you recorded earlier.

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Idle Air Bleed With Stamped Letter


The idle air bleed has a height setting that is set in relation to the travel of the metering rods. If there is a letter stamped on the top of the idle air bleed screw it is considered non-adjustable and is set to a specific height using a special tool that depresses the mixture control solenoid plunger.

The most common setting for the idle air bleed screw is to gently turn it clockwise until it bottoms out then back it out counterclockwise 3.5-4 turns.

If there is no letter on the top of the idle air-bleed screw, it is still set with the height tool, but can be tweaked slightly to alter the mixture control solenoid dwell period. (See Final Settings section later on.)

The most common setting for the idle air bleed screw is to gently turn it clockwise until it bottoms out then back it out counterclockwise 3.5-4 turns.

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Idle Air-Bleed Tool


If using the idle air bleed tool, it simples slips into the carburetor’s triangular-shaped vent slot. Then push down on the rod, depressing the fuel metering rods and adjust the idle air-bleed screw till it just touches the top of the rod.

Mixture Screw Tamper-Resistant Plug Removal

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Tamper-Resistant Plug Removal


The carburetor must be unbolted from the engine before you attempt to remove the mixture screw tamper-resistant plugs.

There are two round marks on the base plate near each of the mixture screw tamper-resistant plugs. Some carburetors do not have these round indentations, if there’re no indentations cut alongside the plugs about 1/4 inch.

Cut an approximately 1/16-inch deep grove from the front of the base plate to the round marks.

Gently use a small chisel to tap out the cut aluminum and the steel plugs covering the mixture screws and discard them.

These are the only plugs that will not need to be replaced after all of the adjustments have been performed.

Seat the mixture screws gently in their bores, then back each out 3.5 complete turns. This is your base setting.

Final Setting
First, let the engine run until it is fully warmed up so the system is in “closed loop.”

After all internal adjustments are correct (the float level, mixture control solenoid travel, idle air bleed and TPS) the dwell can be set. Remember, we are looking for a dwell of 28 to 32 degrees (30 is perfect). This adjustment is made by adjusting the mixture screws.

Adjust each mixture screw a like amount to achieve a dwell that averages around 30 degrees. The dwell reading will bounce, so look for a range of 28 to 32 degrees. If the dwell is lower than that, then the mixture screws need to be turned out. If the dwell is higher than 32 degrees, the mixture screws need to be turned in. The adjustment will be very sensitive so you will only need to turn each screw 1/8-turn or less at a time. This should achieve a smooth idle with a dwell near 30 degrees.

If the carburetor has an idle air bleed screw without a letter on top you can adjust this screw to fine-tune the dwell. For example, while adjusting the mixture screws, if the engine idles better at 28 degrees or 33 degrees leave it there. You can then adjust the air bleed to get the dwell in range (closer to 30 degrees). Adjusting the idle air bleed screw clockwise will raise the dwell; counterclockwise will lower the dwell. Again, this is a fine adjustment so go in small increments.

Dwell Troubleshooting
Dwell not changing? The system is probably in “open loop.” For the computer to go into closed loop a few things must happen. The engine must be at operating temperature, the oxygen sensor must be reading and a specific amount of time must have passed since the engine has been started, usually around five minutes.

Dwell stuck low (15 degrees or less)? A rich command is present and the engine controller is compensating for what it thinks is a fuel flow reduction. Possible causes are vacuum leaks, debris in the carburetor, the air injection system stuck in upstream position or the air/fuel mixture setting not correct.

Dwell stuck high (45 degrees or more)? A lean command is present and the engine controller is compensating for what it thinks is a high fuel flow. Possible causes are a mixture control solenoid failure, stuck or a low float, dirty air filter, oxygen sensor not reading correctly or possible valve seat damage.

Dwell fluctuating with an average reading of below 30 degrees? Again, a rich command is present. Possible causes are vacuum leaks, debris in the carburetor, the air injection system stuck in upstream position or the air/fuel mixture setting is not correct.

Dwell fluctuating with an average reading of above 30 degrees? A lean command is present. Possible causes are a mixture control solenoid failure, stuck or a low float, dirty air filter, oxygen sensor not reading correctly or possible valve seat damage. But more than likely the air/fuel mixture setting is not correct.

There are some special tools that are required to make the adjustments. These include a “Double-D” wrench for the rich stop, lean stop, TPS, and the mixture screws. There is a special ruler used for adjusting the metering rod travel and a height gauge for the idle air bleed screw.

Several companies sell complete sets of the feedback carburetors tools such as Snap-On and Thexton Company, just to name a few.

I hope this helps with understanding the computer controlled feedback carburetor. Thanks for all of your questions and keep them coming. I will try and get to as many as I can. Vette

Photos by: James Berry

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