I picked up this '84 B2000 sundowner a little over a year ago. It ran great but had a clear HG leak so I pulled the head. Ended up doing a complete teardown of the engine due to some combination of very old oil and HG sealer. No new parts aside from bearings and seals. Put it back together, then I made the mistake of letting it sit for about a year before I got the engine back in and tried to start it.
The problem I'm having is that it will start and run ok if I'm revving pretty good, like maybe 2500 (no tach so that's a guess). At that RPM and above it sounds right but if I drop the throttle just a few hundred RPMs it instantly and completely dies. This cutoff point is very abrupt and definitely related to RPMs, like if you keep your foot on the pedal at juuuust the right point the engine purrs along real happy and then just feather it down a hair and BAM it dies. Even if you slam the gas back down almost instantly it usually won't stay running, almost like you hit an on/off switch. I'm hoping this issue is specific enough that I can diagnose the problem without going through all the systems one piece at a time.

The truck has a whole ****storm of vacuum lines, pretty much all the original vac equipment and a weber 32/36 (was this a stock carb for these?) . I know I got the vacuum setup pretty close to how it was but it's possible I misrouted something, there's a lot of little lines.

A few questions:
- I set timing by cranking w spark plugs disconnected and the vacuum advance on. There are two marks on the crankshaft pulley I lined the mark on the right up with the little pin on the timing cover as per my Haynes. is this correct? Could timing cause it to die below this very specific RPM?

- The carb sat for a year or so and I didn't mean to let it sit so I didn't put anything up right. Could it be as simple as a clogged idle jet? Does anyone know at what throttle position the main jet comes into play? Not very familiar with carbs. They way it dies it kind of feels like a fueling issue but it's more sudden than I'm used to.

-Would bad gas do this?

-Aside from the obvious major leaks are there any spots on this over-complicated vac system which would cause it to die when you drop the revs like this?

 

Mine's a 1988 B2200, but I do have a Weber 32/36DGEV. No, the Weber was not stock, definitely aftermarket. Most likely you could cal off all the vacuum ports except for the one to the distributor. Make sure there's no vacuum leak at the Weber adapters, these need to be sanded PERFECTLY flat with wet/dry on a sheet of glass, and gaskets with Permatex Aviation or similar non-RTV sealant. Because vacuum leak(s) could affect timing, maybe set the timing with distributor outlet and vacuum line plugged off.

 

Thanks. Turned out to be a fuel pump issue mainly. It tested out ok at first but a bad ground or something dropped the voltage a bunch, I think what was happening was that when the engine was revving the alternator kicked up the voltage enough to make up for the bad ground/pump which is why it ran ok when I kept the gas on and then died instantly when I let up. Pretty odd. There was also a vacuum leak. Seems to be running good now.

 

Glad that's it's doing better. My 1988 B2200 has a mechanical fuel pump bolted to the cylinder head.