Hi Jim, I'm back. I dug out the '88 D50 manual at work today and got a surprise. The carb you have isn't the same one that I expected. 1988 model year was a major departure from the earlier type, used for many years. While it's an upgrade from the earlier models, I don't have the depth of experience in this one last gasp of carburetion. It was simply a less-needy unit, so I didn't see enough of these units to create a lasting impression. That said, let's get started.
Choke operation and choke breaker condition remains the focus on the cold engine loading. The choke breaker just upgraded from a single diaphragm to a dual diaphragm assembly. The first half is active at all times, pulling the choke open immediately upon cold start by a measured amount (spec is about .100", not much). After coolant temperature reaches 65 degrees F, a thermovalve opens and applies vacuum to the upper chamber, further opening the choke blade (.130", measured at the top edge of the choke blade to the air horn). The two diaphragms are serviced as an assembly (part number MD617170) and come with new castings and springs.
Should you see no resistance to light finger pressure when attempting to close the choke blade, at least the lower breaker has ruptured. After 65 degrees, the second system tags in, so you might see some resistance at that point. Not good enough. You have to have the lower breaker working or the engine might not run long enough to get to the second stage.
Replacement appears to require removing the electrically-heated bimetal choke cap, located to the side facing the bulkhead. Tamper-proof screws will attempt to repel your efforts, possibly making carb removal necessary. The choke breaker shaft runs into the cavity covered by the choke cap and directly interacts with the linkages.
The breaker assembly (I apologize for the lack of visual aids) is located in a direct line from the bimetal cap to the vehicle right (toward the valve cover). It has a single vacuum hose connected to the outer 3-screw aluminum cast cover. The vacuum nipple on the cover has a single vacuum hose connected to it and has a vacuum delay valve cut into the hose about 4-5' away.
You may need to remove the secondary depression chamber to access the screws on the breaker assembly. The depression chamber, then, would be the large gold-colored thing (classic UFO shape) that has a link extending to the secondary barrel throttle shaft. Usually it only takes two screws to release it and it can just hang if it's sufficiently out of the way. A word of warning: Mikuni phillips-type screws are notoriously soft. They round easily. If you can catch any of these with a pair of battery pliers (my pref) or other strong-jawed plier, try to break them loose first. That way, you still have good phillips ends to work with later.
Once the three breaker screws are removed, tap on the assembly lightly and it'll fall apart. Make note of where the springs (I think there may be two) go. Remove the breakers, insert new. Initial adjustment of the lower breaker can only be done with the bimetal choke cap removed. With the breaker assembly screwed down tight, reach into the cap recess and move the breaker shaft to vehicle right (like it would do if vacuum was applied). You will also need to close the choke blade by hand. Measure the distance from the choke blade top to the carb air horn side. If your altitude is below 4000 feet, the factory adjustment will probably be OK. Above that... you might consider opening it another .010" or so.
The second stage can be checked by applying a vacuum source to the outer nipple. This should provide an additional .020" of kick (goes to about .130) and is adjustable by turning a set screw in the breaker outer casting (you'll see it). Backing it out increases travel, increasing blade opening.
Another note on carb removal, should it be necessary. You will need to drain some coolant, enough to get the level below the intake manifold surface where the carb bolts on. It has a coolant-heated base.
OK, hesitation. You say in your last post that it happens any time. Since the truck sounds pretty much undriveable until it heats up, that's where I'll concentrate the attention. Also, you state the problem passes and then it runs better, if not OK.
An immediate hesitatitation with rapid throttle movement is classic... lack of accelerator pumpitis. Your carb has two accelerator pumps. Lucky you.
The pump that you would call normal is the one that shoots a stream of fuel into the primary carb throat each time you open the throttle. A quick opening will produce a somewhat stronger stream, comparted to a slow roll-in. You can test for this by simply looking down the carb throat (engine off) and operating the throttle. Lack of a good, tight stream indicates something less than what you'd like. If it sprays in different directions, you have a blockage in the discharge nozzle. If the spray is erratic, not always appearing to start at the same throttle travel, a check valve internal to the carb is likely not sealing properly. Either way, there isn't much that can be done without carb disassembly. Pump stroke isn't adjustable on these units, either. Could be that the link fell off the throttle shaft-- they're held in place with a plastic grommet (tiny) and a small e-clip. The grommet could have disintegrated, eventually wearing away the clip (just a thought).
The second accel pump is for cold engine use only. It operates up to 65 degrees of block temperature, then shuts off. It's operated with engine vacuum, plumbed through a thermovalve that passes vacuum only to that temperature. An anaology: Imagine you're at a pinball machine. The ball rests on the plunger, waiting to be pulled back against spring pressure. Your hand in this case, becomes engine vacuum. Pulled back, the ball follows the plunger and waits. Releasing the plunger sends the ball shooting into the game, never to return again.
Engine vacuum (cold engine) is applied to the backside of a spring-loaded diaphragm. It causes fuel to be drawn into the chamber on the other side, where it sits, idle. A change in engine vacuum (acceleration) causes engine vacuum to drop, releasing the spring. Problem is, this applies only to cold engines and I'm not sure it applies.
All the engine sooting you've had might have had ill effects on the oxygen sensor. On your system, a failed sensor usually isn't detected by the feedback system. Rather, lack of a signal appears to be a lean mixture and may cause additional fuel to be dispensed.
Carb bowl vent failure can be a source of grief, too. Outside air pressure needs to be prevented from reaching the float bowl while driving, because the pressure differential between air cleaner and atmospheric can literally push fuel through the jets. Sometimes, just removing the air cleaner lid for a test drive is all it takes to tell if you have problems. Runs OK, bowl vent time. Not always, tho. The bowl vent is connected to the largest hose exiting the carb body, snaking its way across the valve cover to the vapor cansiter. With the engine running, disconnect the hose at the canister and attempt to blow through it lightly. If the bowl vent is working properly, nothing happens. Failed, it will be open and you'll flood and kill the engine. One other possibility is that vacuum may be applied to the carb bowl from a rotted bowl vent diaphragm. Vacuum is what's used to operate these units and if the vacuum makes its way to the bowl, it can have just the opposite effect--difficulty getting fuel through the jets. You may note a light vacuum on the hose, easiest with the hose in your mouth (yuk, I know). Ya duz wut ya has to duz sometimes.
I'll leave you with just this quick post for now. While more possibilities exist, they get difficult to test, long-distance. You still have EGR, feedback fuel controls, possibility of exhaust gas reversion from aspirator valves, throttle position sensor, timing (both cam and ignition) and other base carb problems to sort through. I surely wish you luck, Jim. Write back if we get close on something. Ed.