Right then , I've had a look and you are quite right, there is no fuel tap in the normal sense and again you are quite correct in saying that there is no reserve selector either. There is what is described as an Autocock which is the thing you have outlined in red . This device, based on its shape almost certainly contains a diaphragm that switches the fuel on when it is activated by a vacuum from the carburetor. From what I have seen so far, I don't think this thing can be opened up to replace the diaphragm. It seems that it must be replaced as a unit . So , provided you didn't pay too much for the tank and it's got the autocock still in it , that's that sorted then . However, even if this thing is stuck wide open, as I said the carb sill has a float and needle valve that controls the fuel flow into the carburetor float bowl. While I was looking, I was surprised to find that there does not appear to be a choke on the carb , instead there is a Starter Valve so that might be stuck ( see pics ) So anyway what I am basically saying is that a faulty autocock on its own will not cause over revving and spitting flames on it's own . http://i.imgur.com/S8esdqd.png see parts 15 & 7 maybe this thing is stuck . Also as previously mentioned by Tango, have a very good look at part 13 , this is the main diaphragm, check it for holes .http://i.imgur.com/gL3SwDJ.png