What I did when I was starting with a 2 cycle...went to a local auto parts store, bought a couple sections of 1 7/8" muffler/exhaust tubing, cut pieces at 9.25, 9.50, 9.75 and 10.0, tried each of them until I found the one that made my motor the most happy at any given track I was running it at, of course plug checks and checking color inside the flex are mandatory, to try to avoid running it too lean and possibly sticking the piston, or running it too rich and ending up fouling a plug...and need to mention I only ever ran a kbdm Buller pipe on my kt and a kb06 Buller on my open...