This is a lean mixture/air-leak symptom.
How old is the bike? Rubber ages and seals start to leak.
1) Check to see that the rubber boots and clamps are sealing on the carbs, v-boost and inlet manifold joints. Make sure the inlet manifolds are tight and not leaking between the manifold and the head. Liberally, spray water on these joints. If the engine RPM goes down, you have a leak.
2) Incorrect idle mixture setting or blocked air-jets. Adjust idle screws and do shotgun clean on jets.
3) Hole in one of the coasting enrichment diaphragms or perished (flabby and stretched) diaphragms. The coasting enrichment diaphragm moves a brass fitting through a tube in the carb housing to restrict or enhance off idle/on idle fuel mixture. This can bind if dirty or corroded. Remove, inspect, clean and replace as necessary.
4) Hole in carb slide diaphragm or perished (flabby and stretched) diaphragms. Remove, inspect, clean and replace as necessary. Make sure slide diagram covers are correctly installed and not leaking.
5) Long Shot. Incorrectly installed slide springs where the spring does not seat around the prongs on the underside of the carb side covers does also cause this.
6) Air leak from the rubber caps that cover the vacuum inlet tubes on the lower manifold. These are the ones you fit your carb synch tool tubes to.