Alan,
What is the year and model of your bike? Early Americas would hit the rev limit at around 7100 rpms while the Speedmasters would reach 7500. It sounds like you are hitting the limiter though like roadworthy stated. I've had the same happen to me at times where the rev limit appears to change a bit.
Since the only thing you have done is drill a couple of holes in your baffle, I don't think it is fuel related but make sure and check your fuel filter. Don't forget, as you reduce restriction in your exhaust, more air will flow and a larger jet size will eventually be needed. Are your snorkels still installed? If they have been removed, I would rejet.
