I have a 1989 Johnson 70 HP two stroke. When I bought the boat earlier this year I replaced the 3 spark plugs with Champions. The boat seems to run fine because I've never had it run with the old plugs. After doing some reading I found out that the proper plug is NGK. Am I losing gas mileage, power, etc?

Assuming it is running ok, you probably would not notice the difference if you changed them to NGK but I would put in NGK at the next plug change.

It's not always the prig manufacturer that makes the difference. Try using a hotter running plug from the motor manufacturer recommendations.

Not sure where you saw NGK as the recommended plug, Champions have always been the ones I've seen recommended, and the ones I've always run. NGK 's are the ones I use when I can't get the Champions.

Johnson/evinrudes used to engineer their outboards around a certain plug(champions back then not sure now) and what i was told by an old omc guy is thats all youw anna run to get the best possible performance but any plug brand will work. I put ngk's in mine once and i lost about 2mph and i had to adjust my idle and sync my throttle just with different plugs...switched em out a month later..

Not sure where you saw NGK as the recommended plug, Champions have always been the ones I've seen recommended, and the ones I've always run. NGK 's are the ones I use when I can't get the Champions.

Shoulda read this before my post...in 89 they were champions listed in the owners manuals for johnnyrudes...

I put in the NGK's. Made a difference. Hole shot is better, smoother idle, quicker starts, and gained 3mph. Gas mileage seems better too. ;-)