Try a hair dryer to heat the boots, should help soften them a bit. also spray some penetrating oil./wd40 etc around the boots and carbs, let it soak a while.
Other thought is to pull the air box and observe the slides in action. It will hesitate with the air box off but should still rev and run smooth, I'd suspect one of the slides is sticking a bit or fluttering from a bad diaphragm.
