In most cases, the boiling has issue on the early Grizzly 700's was caused by a faulty rollover valve. Yamaha did have a recall or service bulletin about it, but the free repair time period was over years ago.
You can either replace the rollover valve, remove the valve altogether and use rubber tubing as a vent, or put a drill bit through the valve. The valve was updated and any valve you order now is the newer style.
In some rare instances, the boiling gas issue was still present even with the new rollover valve. I personally believe the best option is to remove the rollover valve altogether (delete it), and run just a vent hose like every other ATV before did.
I called Yamaha about this. They had my dealer change the vent for free. I asked if I could just have the part and do the work myself. They said no. Pretty good customer service since my bike is an 07 and they did this for me after I bought it used in 2017. Might call Yamaha about it. The boiling gas issue scared me a little.