Quote Originally Posted by 350for350 View Post
It's hard to tell what may have been done in the past. You use a smaller jet size for increased altitude. That way you get less gas to mix with the thinner air.
Hello, just as an update incase anyone googles this with the same issue. Both my trikes always have black, sooty plugs. Both came originally with 105 main jets stock (that's what Bikebandit and a couple other sites list it as). I bought new main jets sizes 98, 100, 102, 108 and 110. I first tried the 98 on the '81 ATC 200 completely stock (one without the new exhaust). Idles fine, no issue. Once you're at WOT, it revs in variables and not steady (lean). I turn the mixture screw to richen it up a tad and it sounds great. Check the plug, black and sooty no different than before. I'm starting to think perhaps it's something electrical. Both trikes will ride all day as is, but every couple rides the spark plug will foul, replace spark plug, and can continue riding. Perhaps it's just because they are almost 40 year old trikes and the spark just isn't big enough on either of them. Any thoughts? Would love to hear some advise, I love these trikes to death and replacing spark plugs is cheap but I don't want it to hurt my engines...