1986 XR200 Been all through the carb. Clean as a whistle. Clean air filter with no snorkle. A few extra holes drilled into muffler end cap. Good new gas. But still some small what sounds like pre-detonation just off idle in the 1/8 to 1/4 throttle opening. Very annoying when negotiating tight trails. Once I throttle past that it revs up nice. Any ideas?