the current weber i have on my engine was displaying similar problems when first installed. when coming off throttle, it would stay 'stuck' at 1800 rmp. a quick kick of the pedal would get it to drop to somewhere between 800rpm (where the idle was set) and 1200rpm. this was all without a return spring, so i picked one up and it helped a little bit, the idle would return to 1200rpm, but wouldn't drop to 800rpm where it should be.
i discovered the shaft that the cable rotates when you press the throttle (which opens the butterflys) is worn, and has a little bit of freeplay in it. i would fix it, but it doesn't affect driveability so i've just left it as is. :bash:
disconnect the accel cable from the weber, and play with the mechanism that rotates the botterflys. (that's how i found my problem)
dF