Bone stock '91 B2200. 156k miles. Healthy engine, correct timing(ignition and cam timing), great compression. Recent new valves/seals/hydraulic lifters. Burns no noticeable oil between 3k mile oil changes. All emissions equipment are still installed. Recent fuel filter(automatic trans)Assumed to be original fuel pump. The truck has been in pretty much daily use with no issues until about 2 weeks ago when the weather in western Washington state started to dip below the 50 degree mark in the mornings.
The truck will start cold just fine, enter cold idle a little over 1k RPM. After a minute or perhaps a little more, as it comes off of the cold map, or cold idle, it will enrichen the fuel mixture and stall the engine. Black smoke can be seen exiting the tailpipe.

If you rev the engine, it will stay running, but will send more black smoke out along with some black soot mixed with moisture in the tailpipe. This condition completely clears once the engine has reached normal operating temperature, and the truck runs fine from there on out. The spark plugs are not carbon fouled, as i believe the rich condition happens briefly enough that after warmup, the normal combustion process clears them up. The truck does have a single wire OBD0 O2 sensor which i have not checked. If i cold start it during the daytime and the outside temp is 60 or above, it does not seem to exhibit the problem at all. The only check I've done so far is to verify voltage at the choke heater.
I'm getting around 20MPG mostly city driving. Wondering if anyone has any insight, as this issue occurs only during a very specific set of circumstances.
Thanks in advance!

 

Check your vacuum diaphragm for the Choke Pull-Off.......should be the diaphragm (all metal enclosed on the stock carb) that is closest to the engine valve cover. To test it, with the engine OFF, just remove the vacuum hose and attach a 12" length or so, and suck on the hose to see if the device pulls on the choke linkage as it should. I will also further check a vacuum diaphragm when sucking on it by placing the tip of my tongue over the end of the vacuum hose to make sure the diaphragm doesn't "leak down" once it starts to pull the choke linkage.

The Choke Pull-Off is supposed to open the choke flap a designed/determined amount (it can be adjusted) once the engine starts, and therefore creates a steady vacuum that constantly keeps the choke flap opened that designed/determined amount. As the engine warms up, the choke flap is slowly opened up until it reaches it's fully opened status.

If the Choke Pull-Off Vacuum DIaphragm does not respond to this test, then it should be replaced. Let us know what you find.

 

Here is a site you can look at also......

https://www.carburetor-parts.com/test-pulloff

 

Excellent. I appreciate the info and I will resume troubleshooting tomorrow morning.

 

I don't know if a 1991 B2200 from the western Washington area has EFI or is carbureted, couldn't really tell from your posts. At first I suspected EFI, but from MeB2000's posts I guess carbureted, so make sure that the wire from the carb electric choke heater is plugged into the rear of the alternator.

 

Yeah I forgot to mention that it's carbureted. Guess that's pretty important. I keep forgetting what year the EFI trucks came around. The carb heater is receiving proper voltage from the alternator and seems to be functioning properly.

I applied vacuum to the port following the instructions in the video posted by MeB2000 on the choke pull-off and it is working properly as well.

Although I have only owned the truck for a little over 1 year, I have known the original owner and the truck since it was new. I know for a fact that as soon as ethanol began making its way into pump gas that's what was going into the truck. I have been running non ethanol but I'll bet the years of pump gas might be taking it's toll. Plus it sat for a year with that pump gas in it before I got it. Might need to take the carb apart.

I just got done replacing the suspension on my Civic so I don't need to daily the truck right now.

 

He did verify that there was voltage at the "choke heater" so I assumed he had a carbed version.......with the overly rich condition disappearing after the engine gets to operating temp, I would think that it is something related to the choke operation on the carb.

 

I went back with another vacuum pump because i wasn't convinced that mine was not damaged internally. Turns out that there is quite a bit of bleed off on my choke pull-off. It is closing the butterfly when it isn't supposed to. I have temporarily disabled it by plugging the vacuum port on both it and the manifold, and manually keeping the butterfly open. The truck will not stall as it has been recently. I will replace it and call it good. Thank you guys for the responses and the help!