Mikuini's have a port for vacuum advance that works well with the factory Nissan Vacuum pot (limits pulses to the diaphragm). I don't think Webers have the connection. My 40DCOE's don't. I recurved my distributor for the Webers, and that really perked it up!
If you want a cheap alternative, get the 79 280ZX Distributor. It's curve really helps that off-idle bog you get with more cross-sectional area.
Otherwise, get a pro to do it. Rebello or Don Potter. They ain't cheap, but they produce first-rate work.
Mine was recurved in 1990 and it was $350 then. Like I said, you might want to try that 79ZX distributor first... If indeed the advance curve is what's causing your problems. You loosen up the AFM flap yet?
Taht procedure is in the archives. If you are having a delay in response from when you tromp the pedal waiting a sec then all **** breaks loose, it can be greatly helped by doing the flap mod.
Basically there is a geared wheel in the AFM and a coiled spring. You loosen the tension of the spring by slackening a holding fixture on the gear, and relaxing the spring tension by rotating the gearwheel one, two... teeth and then refastening it.
This lets the flap move easier, and lets the engine run a bit richer earlier, helping response. This will not pass smog. It is not a legal street mod for an emissions-controlled engine. Without an O2 sensor on the motor (later ZX's) you can melt the cat doing this as you run slightly rich all the time on the earlier Z's. On a ZX with an O2 sensor, there is no detrimental effect, other than increased response and the emissions thing... The O2 sensor will re-trim the fuel below 3500 rpm and less than 60% throttle anyway.