cant adjust client timeout calling webreference

i call web service as web reference (not service reference)

srvFabGis srv = new srvFabGis(); srv.Timeout = 10000; //10 second

The code above works but if do time out greater than 1 minute it doesnt work

srvFabGis srv = new srvFabGis(); srv.Timeout = 600000; // 10 minutes

time outs at about 50 seconds

What is the problem ?