2005-2009 Honda TRX400EX/TRX400X Service Manual, Page 51Get this manual
MAINTENANCE
SPARK PLUG
Clean around rhe
spark plug base with compressed air before removII1g rhe plugand be sure rhat no debris IS allowed to emer mto rhe combustIOn chamber
Disconnect the spark plug cap and clean around the spark lug baseRemove the spark plug
Check the insu lator or cracks or damage, and the elect rodes for wear, fou ling or discoloration Rep lace he plug if necessary
RECOMMENDED SPARK PLUG: Standard: DPR8Z (NGK), X24GPR-U (DENSO) For extended high speed riding DPR9Z (NGKl, X27GPR-U (DENSO)