It's possible you have bad fuel in the tank. The stuff goes stale over time, more so in recent years than decades back. The process is accelerated if you left the tank mostly empty. As the fuel level goes down, the surface area of the fuel goes up, so evaporation can happen more quickly. (Never store a bike for any length of time without topping up the tank.)I've got a '98 zx6r. I had it stored for the winter for about 2 months and I just tried to start it the other day. It started for a few seconds and then died. After that, it wouldn't start again. I tried again today and it started up with a backfire and then died again. Any idea what is going on?
Along with that evaporation, as the temps cycle up and down over night, the pressure changes in the fuel tank will draw in air each night. Cold, damp air. When the sun comes out and heats up the tank, the air pressure increases, and the air vents out to the atmosphere. But the air doesn't get hot enough to evaporate the moisture that came in with it. The water builds up over time; and your motor doesn't run on water.
The other area that can be a problem is the float bowls on the carbs themselves. That's a very small supply of fuel, to support sudden changes in demand.... since it's a little bit of fuel, when it goes off, it doesn't take too long to clear it out of the carbs -- seconds suffice.
If there is sufficient time, the fuel in the float bowls will all evaporate out, leaving behind the heavier parts of the fuel, which will become thicker, and thicker, and harder to burn.
If you have cruddy fuel in the bowls, and water in the tank, it can do what you are describing is happening to you.
Try some Chemtool B-12, or Techrolene added to your gas, and see if that clears it up. DO NOT mix this sh*t stronger than recommended.:crazy