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Carbed car suddenly ran lean


Active member
Oct 4, 2019
car: 1982 244 B23a/m46 with Pierburg DVG carb, 123 electronic ignition

What happened:
Since I have done some major maintenance on the car I have driven 400 miles with no issues. Tonight however, I was driving up a hill (well, kind of a mountain, I live at 5200 ft above sea level and the mountains go up to 10k...I was probably at 7) switching between 2nd and 3rd gear depending on the rpms in the m46 making sure not to lug while still maintaining ascent velocity. After descending the hill using a combination of engine braking with 2nd/3rd gear and normal braking, I noticed a rough idle like my car was going to die. The rpms were reading a lot lower than normal. My VDO water temp, oil temp, and oil pressure gauges were all looking good and in range of what is normal for the car. The alternator was charging and no battery lights or low volts were showing on the cig outlet voltameter.

I flipped on my wide-band AFR and I was getting super lean read outs---like 17+. The car is normally in mid 14s at idle for reference. These strange read outs were at idle and not while engine braking which normally will show lean anyway as air gets pushed through the motor.

Pulling the manual choke did not do much to change these readouts which is strange because usually it makes the AFR display excessively rich. It seems like modifying the state of the carb was not doing anything. While applying pedal to 2k rpm and above I could get the readouts back to the stoichiometric range so to get home I pushed the choke back in and just kept my foot on the pedal to increase the idle while sitting at lights. The car would have stalled without this for sure.

Recently, I have not done anything differently with the car recently except for fill the new gas tank all the way full with gas for the first time. The car has a new fuel tank on it and the fuel is received at the carb via a new mechanical fuel pump. The mechanical fuel pump has a fuel filter in between it and the carb, but not one before. The fuel tank has an adapted sender on it that works but I blocked off some additional holes as it does not have a return fuel line. I wonder if there is a fuel tank breather issue....

Vapor lock wise: The outside ambient temps were low as I was driving at night and the car has been driven recently much harder (without a full tank) going up hills much steeper than tonight's. I have put the car through rounds of testing in the mountains to make sure vapor lock did not occur over 400 miles or so of this type of driving.

I don't know if vapor lock occurred (this does cause rough idle and lean AFRS) but I was able to raise the rpms with the pedal. The car did not feel especially underpowered during acceleration. I was just getting a really choppy idle and the car felt like it was going to die.

Carb wise, everything is new in the carb except for one piece which controls the movement of the float. (I have a new float retainer to install, the old one was doing the opposite and flooding the carb) I was at a steep angle tonight, but I have been at steeper angles in the past without problems.

Tomorrow when the car is cool in the morning I will start the car and look for the issue to see if vapor lock did actually occur. If AFRs come back to normal I will assume that is what happened. It's just odd though because I have been at hotter temps in traffic, at higher altitudes, and with more load on the car during my testing.

Things I fear: I know valves typically don't get stuck on these cars, but worn valve springs do break. What else could it be?
wacky AFRs


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So was the engine still running well off idle at part throttle, full throttle? Just running lean at idle?

That would be making me think that either some idle circuit jets have gotten plugged in the carb somehow, or there's an air leak somewhere past the carb. At idle you're not moving much air, and the intak vac is highest, so any leak has more of an effect.