On a Tanaka site I read that they want the tank to be slightly pressurized, its around 2 lbs. This is done by the gas cap, it has a check valve built in the cap, lets air in, but wont let it out. Tanaka says to check pressure in tank, shake tank a little, let it set a minute, pull off the fuel line to carb while pinching the line and hold line above the tank, fuel should flow out, or loosen the cap and listen for a hiss. Tanaka says the engine will still run ok with out pressure but top end will suffer. So when I made my 1 gal tanks for my gp 460,s I went to the local lawn mower shop and picked up check valves and 2 hole fuel gromets,3 hole gromet if you use a primer bulb from any echo weed wacker and installed them In my cheap 2.66 gas tank. My top speed dont suffer!