Motorcycle Repair: How to troubleshoot this problem?, octane fuel, virago 750

Question
My bike (1993 Virago 750) runs like crap on anything less than 94 octane. Even premium 91 octane fuel - rough idle, loud backfire on down shift, 3-2-1; stalls out occasionally at stop sign. Thought it was dirt or rust in carb. Tried carb cleaner first. Ran well, til the next tank of gas. Tried fuel treatment. Ran well til next tank of gas. Tried 94 octane. Ran well. Back to 91 octane. Same problems. Tried 94 octane again. Ran well. Can't see how the problem is rust or dirt in the carb? What steps to troubleshoot this?

Answer
Steve, Ive run into a similar problem here in Kansas. The fuel we are getting has an alcohol base and will not work well in some bikes.Try to stay away from any fuel like "flex fuel" or gasahol. This may be one of the califonia models that will only run on premium fuels. Also try new spark plugs. Pull the existing plugs and install new ones that are one number lower--Example  B8ES go to a B7ES this is one step hotter and should help you burn the fuel better.