Originally Posted by Locozodiac
Personally have forgotten to open the air valve several times while sibbing for hours and the engine didn't stop once!! Probably Tohatsu tanks/engines don't have this sort of issue.
I think that is more to do with the size of tank/engine you have than its brand. Possibly combined with the effects mart talks about relating to heat.
I've managed 3 hours with a 20HP yam, a 25L nearly full tank before the engine started complaining. By that point the problem was obvious when you looked at the tank as the side walls had been "sucked in" so they were slightly concave. I'd probably used less than 10L of fuel. Small engines use less fuel therefore last longer before they create sufficient vaccum. The 25L (ish) plastic tanks are quite flexible so deform which "reduces" the amount of vacuum formed. I've seen tanks both "sucked in" and "blown out" from temperature changes. If yours doesn't do this perhaps there is a small leak somewhere.