Apparently my truck - '87 D100 - 5.2L - 757 auto - has multiple personalities.

Situation: I go out to my truck in the morning, outside temp is about 0 degrees C. Pump the pedal once, turn the key, truck starts perfectly. The choke is on, idle speed appx. 1500 RMP.

Scenario #1: Got up on time, so I can afford to let the truck warm up ...15minutes later... Kick off the choke, idle speed falls to 650 RPM in park, then 500 or so in drive with my foot on the brake. Truck is a DOG! no low end power, throttle response stinks, and truck struggles to regain power after a shift.

Scenario #2: Slept in... Start the truck, let it run for 30 seconds or so, kick off the choke. Idle speed drops to 1000 RPM in park, then 650 or so in drive with my foot on the brake. Truck runs FANTASTIC! Plenty of bottom end, great throttle response, shifts like butter. This continues right through normal opperating temps, until the engine is shutt off. If I re-start the motor right away, the truck is back to DOG!

I am thinking timing, or carb adjustments are required. Since I have very little experience with carbs (outside of small engines), and I don't own a timing light, I really can't verify either. Also, the truck will occasionally diesel when I shut down.
Anybody have any suggestions?

 

The dieseling could be timing advanced a little too far and/or idle speed too high.

The other sounds like the choke butterfly isn't opening all the way when the engine is warmed up or is coming back on when it shouldn't. I'm not familiar with the factory choke setup as my '86 has a Holley w/electric choke but a friend with three 318 Dodge trucks from '72 through '83 said they are a PITA.

When it does either scenario after it warms up pull the air cleaner with the engine running and see if the choke butterfly is closed. If so your choke is not pulling off like it is supposed to.

 

So I checked it out and it seems that when the truck is running well the choke is not fully open. So, now what?
I guess that I should probably try to fix the choke issue, but that doesn't solve my problem, it actually makes it worse, since that's the only time the engine runs properly.
Is this a timing issue then? Or something else all together?

 

How does the truck run with the truck fully open?

If the truck runs best after it's fully warmed up with the choke partially on something is leaning it out. Probability number one is a vaccum leak possibly from a vacuum hose pulled off or cracked, loose carb mounting nuts, bad base gasket, ruptured vacuum diaphragm, leaking intake manifold gaskets, broken vacuum fitting, etc.

 

if choke is not coming off all the way it can cause it to diesel because idle is still on choke cam,check to make sure choke resistor is gettig power,it comes from oil pressure switch.