They're hard to needle.

Bob Fulwider, owner of Bob's Hobby Shop in Orlando, FL, flies a Hangar 9™ J-3 Cub with a Saito 60Twin. He says, "4-strokes are different to needle than 2-strokes. With a 2-stroke, you adjust the needle valve based on sound. I always look for a good smoke trail as an indicator of a good adjustment on a 4-stroke. Of course, using a tachometer is the best way to get a good adjustment, for both 2-strokes and 4-strokes."