Fuel tank pressure

After riding for about 10-15 minutes, I start loosing engine power and it will eventually stall. If I stop and open the gas cap, there is a good amount of pressure in the tank. The engine will start and run fine for another 10-15 minutes of riding. If I leave the gas cap cracked open, it runs fine. It seems to do it regardless of petcock position. I don't see a tank vent anywhere.

Bike is an 82 750 ltd

If it's like a kz900 or 1000 it under the tank cap.
Pop it open and look under the cap you should see in the center a little hole

Take the cap off, disassemble, and clean with WD40.

jackleberry wrote: Take the cap off, disassemble, and clean with WD40.

+1
On my 750 the vent hole is not noticeable but on disassembly you will find a series of little channels that allow air in around the keyhole.

+2. Your tank vent in the gas cap is plugged. My guess is you are creating vacuum not pressure in the tank.

Thanks everyone, rebuilding the cap fixed it. There was some nasty gunk in the tiny air passages. The tank was in a vacuum with the engine running, and would then pressurize a few minutes after shutdown due to heat soak. It would literally blow the cap open when unlocked.