1. Gummed up jets and air passages.
2. stuck float or float needle or debris in float needle seat.
3. See #1.
You'll need a carb cleaning and since your bike is almost 20 years old, you should rebuild them.
Rebuild kits are about $12 per carb. I don't recommend this job for people who don't know anything about it. It's not hard but amateurs find it complex.
If you do decide to do it, don't attempt it without a shop manual and the phone number of someone who knows how do it. You won't need any special tools except for a float height gauge or vernier caliper. Also, don't disassemble all of them at once. Leave one for reference until the others are done.
A lot of people screw this up.