GW look at the inlet side of the pipe and if it is not completely under water it is not in siphon mode. Don't try and put any body part near an 8in siphon as it may well kill you. Our 8 in siphons can put out that much water easily in non siphon mode. If you have to try to stop it without causing an air leak then do so on the inlet side by covering with a sheet of plywood using attached ropes.