There is a check valve on the tank under the black cover, there is a recall on them ask your yamaha dealer about the 2007 grizzly 700 gas tank vent valve.
How To Fix Gas Boiling On 700 Grizzly 2007?
Sounds like you need a metal heat shield in between the motor/exhaust and the gas tank. Plus some heat shield tape from jeg's. Around the exhaust.this would be my guess.