I'm having a performance issue with my truck. When the engine is cold: It takes about 15 pumps of the accelerator to start. It idles smoothly and doesn't stall. It hesitates often (big lurch) and occasionally backfires either through the carb or the exhaust. When the engine is hot: Still idles smoothly. Doesn't backfire; hesitates very rarely. Engine is much less powerful when warm than when cold. In particular, in the middle of the throttle range tends to yield even less power than when I release it a little. Pressing the accelerator to the floor will cause a downshift because of that silly GM downshift adjustment cable, but still not much power. Big jerk back when it upshifts again and slugglish acceleration resumes. Also, engine seems less powerful when it is humid outside. It never misses, sputters, or runs roughly.
Experience with this problem: Sounds like an overly lean fuel/air mixture, right? The carb does whistle faintly during idle. I can't find any air leaks, and have disconnected and plugged all vacuum hose fittings -- still no change. The carburetor is new (remanufactured Champion 2SE). I've cleaned the carb -- helps a little, but not very much. The choke is working fine. Both barrels are opening. I did replace the gasket and adapter plate between carb and intake manifold, and tightened the carb down just a little harder than specified in spec. Verified with timing light that timing is OK and distributor advance is working. I've read that low fuel pressure could cause similar symptoms, but I don't have other common lfp symptoms, e.g. the engine isn't overly difficult to start, doesn't EVER stall, and doesn't sputter, idle roughly, or run roughly. Every book or article I've read states there's a mechanical fuel pump on the engine on carbureted models, but mine is different. There's a plate over the outlet for mechanical fuel pump. There's an electric fuel pump in the fuel tank, but not the same one as on the fuel-injected models. (The pump doesn't make a whirring noise when it runs, but instead a constant "tapping" noise as the diaphragm pulses.) I don't know if this is a factory pump or not. The engine temp stays right around 150 F, as in spec. All other vital signs seem fine.
This is driving me crazy! If anyone has any suggestions, I'd be very grateful and will return the favor any way I can. Thanks in advance.