The spring should hold the nozzle firmly against the venturi.
If the nozzle spigot and/or the venturi hole are worn from rattling about, performance and tuning are compromised.
If the venturi is loose in the bore, it will rattle upon the spigot as well, and any daylight around it adds another performance and tuning compromise.
If your low-speed needle is dialed in fairly well when warm, then find a safe straight-a-way where you can cruise at a steady speed above thirty mph (48 kph). Turn the high-speed knob in leaner a few clicks at a time, waiting several seconds each time for the circuits to equilibrate.
When the exhaust note goes tinny, or the motor begins to falter, back the needle out in the same fashion, counting the clicks until you find the limit on the rich side. Then turn the needle back in half the number of clicks.
At this point it is usually best to pull over and re-adjust the low-speed. The circuits over-lap, and the low-speed actually controls air at higher RPM. So tuning is a matter of tweaking the needles alternately for a finer and finer adjustment. Your return trip on the straight-away should require just a couple of clicks to find the high-speed sweet spot.
If accelleration problems persist, then inspect for the problems mentioned earlier, as well as borewear from the throttledisc, etc.