The ornamental design for a decorative motorcycleexhaustpipe, as shown and described.
FIG. 1 is a perspective view of a decorative motorcycle exhaust pipe showing my new design in position of use on a motorcycle, which is shown in broken lines for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is an enlarged bottom plan view thereof;
FIG. 3 is an enlarged top plan view thereof;
FIG. 4 is an enlarged left side elevational view thereof; and,
FIG. 5 is an enlarged right side elevational view thereof.