The ornamental design for spark plug igniter, as shown and described.
FIG. 1 is a side elevational view of a spark plug igniter showing my new
design; FIG. 2 is an enlarged portion perspective view thereof; FIG. 3 is
an enlarged plan view thereof; FIG. 4 is an enlarged side view thereof;
and, FIG. 5 is an enlarged cross-section of the threaded portion as taken
in the direction of arrows 5--5 of FIG. 1. The broken line showing in
FIGS. 1-4 is included for the purpose of illustrating environmental
structure only and forms no part of the claimed design.