Seattle Municipal Code 11.57.280:

No person shall modify the exhaust system of a motorcycle in a manner which will amplify or increase the noise emitted by the engine of such vehicle above that emitted by the muffler originally installed on the vehicle, and it shall be unlawful for any person to operate a motorcycle not equipped as required by Sections 11.84.060 and 11.84.080 or which has been amplified as prohibited by this section so that the vehicle's exhaust noise exceeds ninety-five (95) decibels as measured by the Society of Automotive Engineers (SAE) test procedure J1169, (May, 1998). (RCW 46.37.390(3))However, you can still get a ticket for $124 for excessive noise even if the motorcycle's exhaust system isn't modified.  According to section 11.84.080 of the Seattle Municipal Code, all motor vehicles must be equipped with a muffler in good working order at all times and in constant operation.

Are motorcycles with loud exhausts legal?

 

A: Seattle Municipal Code 11.57.280:

No person shall modify the exhaust system of a motorcycle in a manner which will amplify or increase the noise emitted by the engine of such vehicle above that emitted by the muffler originally installed on the vehicle, and it shall be unlawful for any person to operate a motorcycle not equipped as required by Sections 11.84.060 and 11.84.080 or which has been amplified as prohibited by this section so that the vehicle's exhaust noise exceeds ninety-five (95) decibels as measured by the Society of Automotive Engineers (SAE) test procedure J1169, (May, 1998). (RCW 46.37.390(3))

However, you can still get a ticket for $124 for excessive noise even if the motorcycle's exhaust system isn't modified. 

According to section 11.84.080 of the Seattle Municipal Code, all motor vehicles must be equipped with a muffler in good working order at all times and in constant operation.