I bought a few sets of the 9EIX's for this upcoming season. Was going to experiment with them some. The old 9EV's I was using were good to me. I did find the price to be a little difficult to deal with, but what the heck. . . .
I run the cheap BR9ES or 8ES. Havent fouled a plug in years but i still change them out every other season for good measure. Just tighten them good and snug after the compression ring is done squishing. If you're fouling a lot of plugs you either need to look at your carb settings or stop trying to run your sled upside down
IMO, the cost of Iridiums really offsets anything you might gain. Unless NGK is sponsoring you and you get them for free (or at a greatly reduced price), you are wasting your money.
I always get 1000+ miles out of Champions.
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