blue sediment in carburator

[txninak@arctic.net]

I have a 1999 Yamaha F50 4-stroke, which I have used for three seasons with no problems whatsoever. To winterize, I add a gas additive to the fuel and run the engine dry. I did not until this year realize I could drain each carburator, so likely a small amount of fuel remained each winter. At the beginning of each season I start out with cleaned tanks and fresh gas.<br /><br />This year, the motor has performed inconsistently, lugging and missing at both high and low RPMs. It occasionally runs well, but only at RPMs above 3000. I have cleaned the tanks, fuel lines and filters, and replaced the gas, with no change in performance. Upon draining the carbs, I discovered a blue crystalline substance that breaks into a powder when you press it. Neither gas nor carb cleaner dissolve it, but spraying cleaner into the carb seems to flush it to the bottom of the bowl. I am assuming this substance is what's causing poor performance. What is it? Where did it come from? How do I get rid of it? And how do I keep it from coming back?