This is a 235, right? Suck on the advance line, see if distributor moves. The vac advance won't really affect performance in neutral. Either your mechanical timing advance system is gummed up, or your carb is screwed.
Mixture screw only affects idle. The main jets/fuel system inside carb controls above-idle mixture. Could have a vacuum leak somewhere. Check sparkplugs for carbon fouling.
Check timing with a light. Shoot for about 10-12 degrees at idle, rising to about 35 degrees total by 3,000 RPM. Look for no or erratic spark advance.
How does truck run going down road? I've seen 235s do weird things at idle, but run fine going down road.
chip
