fuel line problem

[nubbiker]

hi there, just bought my first bike gz250 (used) and am having serious issues.
rode fine until i took it out on my first big ride and pushed the bike a bit (accidentally let the fuel run really low)
that evening it started cutting out on me and not starting up again.
eventually i couldn't get it started at all, bike collected by mechanic, he says starter clutch is gone. he replaces worn parts.
around the corner from the place bike cuts out and won't start again. so i watch him
1. take apart the carb, and blow it out including the fuel lines , there was water and sediment in it.
bike starts but stops again
2. adjust the air / petrol ratio, moved the diaphragm thing up one notch. to give richer fuel.
bike starts and carries on until you stitch it off again
3. check sparking and electronics
no problems there.
4. replaces choke with a manual one which has a plunger thing on it my one didn't have (instead of connected to my handle bar)
still problem.
5 finally replaces spark plug.
and it works every time until it rains on the bike. it starts cutting out but i can start it every time
the next day ... today. i refill with fuel and around the corner the bike cutts and won't start again

so now i've got the same problem all over again minus the clanking of the started motor. the mechanic won't relook at it without more money than i have. does anyone have an idea what i should try next
thanks chris