The Mighty 258 > Fuel Systems.

Whispering to the Feedback BBD.

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Sometimes you still get stumbling, hesitation, surging, and bogging on a feedback 258 even after you've gone through all the vacuum lines, sensors, switches, motors, and other equipment related to the fuel feedback system and ECU. Many owners bypass the system by locking the stepper motor in a permanently rich "cold engine warmup" position, but here is how to actually fix it:

The problem is they are too lean. They were right on the ragged edge of lean originally (like all emissions-era carburetors are), and any little thing can push them over the edge. Vacuum leaks are obviously one of those things that makes them leaner, but another thing is ethanol gasoline. 10% ethanol gas burns 5% leaner than pure gasoline, and that is significant. That's like a 4000 foot difference in altitude. It's enough to cause drivability problems like hesitation and stumbling.

The ECU can act through the carburetor stepper motor to richen the fuel mixture in compensation for ethanol fuel, but only during closed loop when it's receiving feedback from the O2 sensor. Closed loop only occurs during part throttle cruise, and there is a few seconds of delayed response time. All other engine operating conditions are open loop conditions where the ECU blindly drives the stepper motor to pre-programmed positions regardless of how rich or lean the result may be. All of these open loop conditions can be too lean on 10% ethanol fuel.

The solution is to enlarge the carburetor main jets by 5% to make it run at the same fuel mixtures on 10% ethanol as it was originally designed to on pure gasoline. Replacing the original #92 main jets with #95 or even #96 jets will do the trick. Carter BBD's use the same jets as Carter AFB's and Edelbrock's, so jets are easy to find and buy.

Another trouble area that can make them too lean is the metering rod adjustment. The procedure outlined in the manual that tells you to adjust the vacuum piston gap using the metering rod adjusting screw before setting the metering rod linkage is NOT how Carter set these carburetors up at the factory, and will almost always cause a lean bog at light throttle. Here is how Carter originally did it:

1. Turn the metering rod adjusting screw on the metering rod yoke counterclockwise until it stops to fully lower the metering rods.

2. Adjust the metering rod linkage as outlined in the manual to bottom-out the metering rods in the jets when the throttle is fully closed.

3. Turn the metering rod adjusting screw clockwise until it stops to raise the metering rods and count the number of turns that it took. Most are 2-1/2 turns. Now split the difference and go back 1-1/4 turns to place the metering rods in a middle position. Carter made this adjustment on a flow bench, but you'll have to do it by seeing how the engine runs. Turning the screw clockwise richens the mixture. Keep it as lean as possible without causing stumbles or hesitation at light throttle.

You will know if you have it set right by watching the stepper motor pin through the top of the carburetor. It will idle with the pin in a preset (open loop) middle position, but the pin will move as you open the throttle slowly to increase the engine speed in neutral. Maintain the engine RPM at 1500 to 2000 for a few seconds and look at where the pin settles to. It should dance around a middle point in its travel, indicating closed loop feedback from the O2 sensor. If the pin seems to pull itself all the way towards the rear and stay there, that is an indication that you went overboard on adjusting the carburetor richer. You should turn the metering rod screw 1 full turn counterclockwise and try again.

You might notice the engine "loading up" rich within a few seconds of cold starting after you've made any of the adjustments I've described. Compensate by increasing the choke initial clearance by about .030". Don't rotate the choke thermostat because it's not the cause of your problem and rotating it will cause leanness elsewhere.

Adjusting the fuel mixture on any engine with a catalytic converter should never be taken lightly. Too rich or too lean will dump raw fuel into the exhaust and melt the converter, sometimes in merely seconds. A melted converter is a plugged converter unless it melts down completely and blows it's chunks out into the muffler. How can a mixture that is too lean dump raw fuel into the exhaust? Easy. It misfires. A lean misfire dumps more raw fuel into the exhaust than running a little rich does.

The final thing I will leave you with is the EGR system. Feedback 258's have a relatively high compression ratio, as high as 9.2:1 on 1984-1991 models. The high compression gives them great torque output, but it makes them prone to knock. AMC fought off knock by using a lot of EGR. You can very easily get bogs and hesitations from having that much EGR, especially if the engine is already lean. Lean engines HATE EGR. Always try to keep using your original EGR valve. The aftermarket replacement EGR valves with the washers under them almost always have worse driving manners than the original valves. The original valves almost never go bad and get blamed far too often when something else is going wrong. If after all else you still have a bog from the EGR, try switching the EGR vacuum source. Most 258's use the "S" port on the passenger's side of the carburetor to operate the EGR, and have the "E" port on the driver's side capped off. This is contrary to most of the U.S. auto industry that used the "E" port specifically for EGR. The "S" port will cause the EGR valve to open the instant the throttle is opened above idle, while the "E" port is less aggressive and waits for a little more throttle opening before opening the EGR valve. Switching the EGR valve to the "E" port will definitely reduce or eliminate EGR bog at light throttle while still providing the necessary EGR at medium throttle to stop knock. Just remember to not chase your tail down the EGR rabbit hole if a lean fuel mixture is actually the cause of your problems.

I hope this helps those of you who have working feedback systems but still fight drivability problems. I fought my system for years until I figured this all out.

AMC of Houston:
And there's also the AMC BBD service letter - says to drill out the bottom of the idle bleed tubes to 0.032 - that in itself cures a bunch of BBD ills.  I do that on every BBD I rebuild; and it works.

I ended up in 2019 doing a complete hose replacement followed by an EGR cleaning, carburetor cleaning and adjustment which dialed the system in 90% of the way and the remainder was altitude tuning and correcting a reversed CTO hookup.

Under my ownership I have never drilled the tubes out but it now behaves completely through the ambient air temp range from -30c to +47c without bog-outs, vapor locking or knocking at its "home" altitude of 2000 feet.

IMO the only proper fix for an EGR is a blockoff plate


--- Quote from: TheBirdman on November 26, 2022, 01:28:41 PM ---IMO the only proper fix for an EGR is a blockoff plate

--- End quote ---

LOL!  IMHO, the only way to properly address all these fuel issues everyone seems to be having lately is the attached pic...

At some point, the lack of decent parts support + time spent trying to diagnose a 40-year-old system that requires every single little electronic doohickey to be working absolutely perfectly should lead everyone down the EFI path, but I guess some people are gluttons for punishment.


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