ORIGINAL: Jester241
that one guy said it could come from the exhaust not having enough pressure to vent the tank. This made me think a bit,but it cant be that because the problem gets progressively worse as the tank gets empty. If it were a vent issue,it seems that it would get better as the tank got lower I think.
Not necessarily.
Since you are talking about the engine quitting when the plane is going straight up, the engine is leaning out more, the lower the fuel gets.
That's because the carb must generate a lower pressure area to get the weight of the fuel up the effectively increasing in length line.
If the muffler is not creating enough pressure in the tank, and/or there is a leak somewhere before or in the carb area, the low pressure seen at the carb may not be enough to pull the fuel up, starving the engine causing it to quit.
I hope you tuned your engine with about 1/3 of a tank of fuel in it, but not much more...
Then once you got it tuned you performed a nose up test to check your settings.
Did you skip this by chance?