Some Alpha trainers had problems with the fuel tank stopper being overly tight. The result was that the tank would split after a little while. Sometimes on the first flight, sometimes later.

A split tank will cause the engine to run like you describe, you can often get it running on the ground, but it goes lean and quits on take off. And, of course, the fuse fills with fuel.

The other suggestions above about the fuel lines, clunk, etc should be checked. (most likely problems, IMHO, stuck or loose clunk, hole in line, reversed lines).

One way to test your fuel system is to pinch off one line, and blow in to the other, then pinch it off, getting pressuraized air in the tank. (do it with an empty tank, btw, and be careful, glow fuel tastes horrible, and is poisoness). Hold the lines pinched, and gently move them around, pushing, tugging, etc to get the lines to bend in various ways. Then releas a line, you should hear a noticeable hiss as air escapes. A better way is to pull the tank and do the whole pressure test in a sink of water. Any air bubbles and you've found your problem. You want to move the tubing because the silicon fuel tubing we use can hide or even seal up small holes if pressed the right way, but they open up under engine vibration.

Oh, also check that you haven't hooked up the lines backwards. If you hook the muffler line to the carb, you get exactly what you describe, a very short run followed by a quick cut off.