Have run out of ideas - 94 wide glide Mikuni carb single fire ignition (dyna) no electronics uses centrifugal advance unit. Bike runs fine pulls strong with no flat spots. When I come to stop it will not return to idle right away, no tack but it runs about 1500 to 1800 rpm, after stopping it will return to idle after a few seconds. Is it the carb or the advance unit?

 

Its most likely an intake leak. Check the seals between carby and manifold and manifold and heads.

 

Intake leak was my first thought also but could not find a leak but will check again just to be sure ...

 

If you can't find a leak, try fattening the bike up a little, when they're really lean, they will do the sane thing

 

Does your throttle have two cables? I think a missing closing cable will do that.

 

plugs look good engine does not burp fart or make any strange sounds runs strong - has return cable for idle and it is working right - this has me stumped for sure...

 

Symptoms point to a vacuum leak - but, if you're still running the centrifugal advance (why, btw???) it's not probable, but, it's possible for the weights to be hanging up and not allowing the timing to retard when the RPMs drop.