85' Goldwing Throttle Return Problem

My 85' Goldwing GL-1200 came back from the dealer with a problem unrelated
to what it went in for. When releasing the throttle to shift gears, the
engine speed lingers where it was and gradually returns to idle after maybe
4 or 5 seconds rather than immediately dropping towards idle speed. Of
course, the dealer has NO idea of how this happened and is doing anything he
can to avoid owning up to it.

I received a few responses to this question earlier, most of which suggested
a pinched our bound throttle cable. I should have mentioned that I had
already been over that and all linkages and return springs are functioning
properly. I've also been looking for a vacuum line that may have been
knocked off, but everything looks good.

Does anybody have a good working knowledge of this system that could explain
how the idle return mechanism works? I've heard a few people mention vacuum
assisted return and/or steel piston type mechanisms.

Please respond to newsgroup only. Thanks in advance.