The Decompressor has nothing to do with fuel.
There are 2 ways fuel can get into the crankcase, either through the fuel pump, if the diaphragm has gone, in which case it will be going straight into the tappet cover through the pulse hose, or, the much more likely and common cause if the float valve in the carburettor leaking.
I would start in the carby, check the needle, check the float for cracks/ full of fuel. even though the solenoid closes off the main jet, if the float is leaking it will overfill the carb and still make its way into the intake.
