So I bought this truck a little over a year ago, and since the day I bought it, it's had an intermittent issue with rough idling. It started out happening only when the truck was shut off and started back up while hot, whereupon it would only idle (albeit, roughly) if the idle speed screw was turned up about 4 turns. Then, a couple minutes later and for no apparent reason, it would go back to a smooth, even idle.
Since then it's slowly gotten weirder and weirder, until I now can expect it to go from good to bad and back multiple times in a drive, run consistently bad for weeks at at a time, or consistently good for weeks. Because of this, I've been working on trying to resolve the issue for the past month.
The symptoms while running poorly:
- Extremely low, rough idle
- Idle drops even further when brakes are applied (seems to suggest something in the vacuum system?)
- Idle drops even further when on a downhill (carb normally only trips up on steep uphills)
- The coolant temp will sometimes get significantly hotter than normal (lean condition?)

I have so far rebuilt the carb, replaced all the vacuum tubing, spark plugs, plug wires, distributor cap and rotor, and fuel filter to no avail. Some of my coworkers at the auto shop I'm at have suggested vacuum tube check valves, but I've only found one and it's working fine, so I'm out of ideas at this point.
Does anyone know what could be causing my problem?? Any advice would be greatly appreciated.

This is what we're talking about BTW:

 

have you done a compression test?

also make sure the choke is hooked up right mine was tampered with and I did not know until I got another carb and noticed when the motor was warm that the top butterfly valve dose not move unlike my old carb where the darned sucker moved when ever it wanted.

 

Looks like a really fun truck, btw. No idea what your problem is though....

 

did you reset the idle fuel mixture back to factory when you rebuilt the carb.

check that the float is working also.

try some of the B12 chemtool carb cleaner in the tank I put it in at half of a tank it dose seem to clean out some of the gunk.

make sure all of your linkage is moving smoothly I have had it stick in the past.

 

I had trouble with mine too. Try a carb rebuild and clean and blow out the carb. Works great now!

 

We thought it might be the choke but the butterfly's opening up pretty firmly at operating temp, we did reset the idle mix screw (which was about 6 turns out before instead of the recommended 4), float's staying in the middle of the sight glass pretty well, and I sprayed a bunch of high temp grease all over the linkage when we rebuilt it, lol.

I'll try a compression though, and I can probably find B12 at a Napa or something right? Thanks man.

 

Could be a HOT SOAK. When you stop, the fuel evaporates in the carb, and its starts rough and the vapor can actually cause a rich condition for a minute or so after start up. Not much to do but rev it after start up to get the vaopr out and new fuel into the float bowl.
BTW, what the hell is that thing on your hood? Unicorn Truck???

 

cb anntenna??