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Author Topic: 2BBD Feedback Mixture Screw Adjustment  (Read 2229 times)

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Offline MIPS

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2BBD Feedback Mixture Screw Adjustment
« on: September 22, 2019, 01:09:56 PM »
Does anyone know the official procedure to make mixture screw adjustments on the electronic feedback 2BBD carbs? I'm running too lean after a rebuild.
Oddly enough, the steps given in the 82 mechanics book don't make consideration for the feedback system and this results in a very hard to tune carb which rubberbands a lot while trying to adjust it. I had a good result by centering the stepper pins, unplugging the stepper motor and then doing the book procedure while verifying the adjustment with the O2 sensor but this isn't a written procedure, so only knows if this will hold up under regular driving.

Offline AMC of Houston

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Re: 2BBD Feedback Mixture Screw Adjustment
« Reply #1 on: September 22, 2019, 02:58:34 PM »
I don't have the procedure handy(its in the MR251 somewhere); but did you do the Service Letter procedure of drilling out the bottom of the idle bleed tubes to 0.032 during your carb job??  That cures a multitude of BBD issues (including perceived idle lean-outs).   

Also, the BBD's are VERY picky about metering rod height adjustment.  Need to be dead-on to spec there.
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Offline MIPS

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Re: 2BBD Feedback Mixture Screw Adjustment
« Reply #2 on: September 23, 2019, 08:01:24 PM »
The rebuild was a chemical wash, ultrasonic clean, new parts from the rebuild kit and everything reset to Carter spec. The tubes were not drilled.
We assumed that the initial lean was the result of the spec reset and people over the years monkeying with it to make it run better with the computer removed.
Now that the computer is back I can kludge a procedure and the engine responds better and the computer seems to be happy (no more hard lean) but I just want to be sure I've done it right.
« Last Edit: September 23, 2019, 08:04:34 PM by MIPS »

Offline MIPS

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Re: 2BBD Feedback Mixture Screw Adjustment
« Reply #3 on: September 25, 2019, 03:29:50 PM »
So basically the alternate procedure we ran is as follows, for anyone interested.

For starters, make sure that the engine is mechanically sound and timed, all vacuum lines are routed and not leaking and that the ignition, computer, fuel and exhaust systems are all in good condition and serviced with the carburetor cleaned and adjusted to spec with a good air cleaner element. Failure to observe this WILL result in tuning issues.

With a cold start (or the CTS unplugged in '82 year CeC's) turn key to ON. Verify stepper motor initializes and moves the pins to the centermost position. Turn the car off and unplug the stepper motor. This will ensure that the computer does not attempt to correct itself while performing the mixture adjustment.
NOW you perform the idle mixture adjustment as per the mechanics book. Use a tachometer, do not rely on vacuum pressure alone. The adjustment doesn't need to be perfect as found in a non-feedback carburetor. Just put it in the ballpark range the computer can correct with. If not obvious, adjust the screws on both barrels at the same time. The manual doesn't mention this.

When completed you can stop the engine, reconnect the stepper motor and start the engine again. Adjustment can be verified using either a Digital MultiMeter (Click here for video)or oscilloscope tapped into the O2 sensor line (Click here for video) and observing a uniform rise and fall of voltage, displayed as the shape of a sine wave on  the scope. This should be observable under practically all RPM ranges as long as the computer entered Closed Loop. Prolonged dwellling at the top and bottom of the voltage range indicates the computer is unable to maintain the mixture in spec and further adjustment is needed. Start over from the beginning by recentering the metering pins as described above.
Operation of the stepper motor can be checked using a clear plastic cover on the air cleaner. Do not attempt to check without it as an open air cleaner will cause a lean condition. As per the manual and design you should see them shift back and fourth in parallel with the voltage from the O2 sensor under all RPM ranges. (Click here for video)
« Last Edit: September 26, 2019, 03:23:32 PM by MIPS »


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