Just heard from my brother that our 98 450S won't start. This summer was running high idle and occasional backfire off full throttle, blew out muffler baffle. Replaced stock muffler, adjusted values and lowered idle with carb set knob. Engine had been rebuilt 2 yrs ago, only 1000 miles on new crank and top end. I don't know if timing chain was changed. Carb is original, 3000+ miles now. No smoke and oil looks good. Recently idle slowed way down and hard to start with idle enrichment lever(choke). Would die until warmed up. Raised idle with thumb knob and ran OK. Last weekend would not run well and gas in air cleaner. Now it won't start. Any thoughts?