I have a US 41 that is in a big Fly Baby. The engine runs fine when on the ground and runs good in air for about 4 minutes and then it dies. Once I get it back down it is extreemely difficult to start again. Could it be that it is getting too hot? It doesn't really seem to be too hot when I check it. This is my first gas engine so I don't know much about them but if anybody has any ideas as to what I may be doing wrong I would be gratefull.

Check two things:
Watch your oil mix, remember it is new (1:32 - 1:40) should do,
Second: With a Tach, as gas engines are hard to tune by ear, find the point where it doesnt get any higher rpm`s and take th ehigh needle 1/4 turn back, to richen a little and try again.

This hopefully should help-

If you've checked everything else, make certain your clunk line in your fuel tank hasn't fallen off. If you have a tygon clunk line, sometimes they slide off the pick up, if they aren't secured with a barb and/or a ziptie.

If you are using one of the older fuel tank conversions with the small black line replace the tank line. These lines can kink and cause problems. Use tygon tubing of the appropriate size. Also check for air in the fuel line after the engine quits to see if there may be a hole in the tank line.