How often should the spark plugs in the HP500efi be changed. Also, I heard that Merc how suggests NGK BR6FS Vs AC MR43T, is this true?

Yes they did change the recommended plug to the NGK.

I have changed mine twice in a 150 hours. FIrst time to correct a fouling problem, second time because of the changed plug recommendation.

Nordic Gary,

The answer depends on the type of boating you do. If most of your time is at cruising speeds, say anything above 2800 RPM you probably can get away with 100 plus hours befor changing. On the other hand, if you do a alot of idling, you may be looking at 50 to 75 hours. A couple of indicators are decreased performance, rough idle, cough on acceleration or mis fire.

I have found no difference in performance or livability between the A/C, Champions or NGKs.

Good luck, have fun.

Steve