'98 WR400 boiling gas, Help!

I'm having problems with the gas in the tank boiling after riding for about an hour. The bike runs poorly when the boiling starts, since the bubbles make their way into the carb.

The previous owner has done some mods such as installing an IMS tank/YZ seat, YZ timing and removing the exhaust baffle. The bike runs great for the first hour, so I haven't messed with the jetting yet.

My high altitude riding lets gas boil at a lower temperature, but this is not normal. I'm running with premium pump gas, nothing exotic. Has anyone else had this problem with an IMS tank, or is my bike running too hot for other reasons? Any help would be appreciated. Thanks

I have read other posts on this topic and the fix seems to be to remove the check valve that is in line on the tanks vent hose.

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