40 CFR 205.173-2 - Tampering.
The manufacturer must include the following statement pursuant to § 205.173-4 with each product of that category the manufacturer distributes into commerce:
Federal law prohibits any modification to this exhaust system which causes the motorcycle to exceed the Federal noise standard. Use of the motorcycle with such a modified exhaust system is also prohibited.
Acts likely to constitute tampering include removal or puncturing the muffler, baffles, header pipes, or any other component which conducts exhaust gases.