If the spark advancer isn't sticking...
Recheck the spark timing.
If that is all good, then it is back to mixture.
It's supposed to idle on the rich side. So one or more cylinders is too lean just when the throttle is return to idle position. An air leak could certainly cause this.
The 77-78 stock exhaust was higher pressure than any aftermarket. This helped to settle the engine back to idle speed without a lot of mixture enrichment. This was EPA influence in the drive to reduce idle emissions.
Anyway, a low pressure pipe changeover makes the 77-78 550 run leaner across the whole throttle range, and hotter, too. Returning to idle, which should be over rich on theses bikes, would take a tad longer to stabilize and cause the hanging you describe.
If you can't find any problems mentioned above, you may have open the ims more to gain back idle enrichment. Corroborating a lean mixture diagnosis, would be snap throttle operation. A too lean mixture won't allow 1/2 throttle snap under load from idle rpm without engine wheeze/stumble.
Cheers,
