Centaurus,
I'm guessing but, as the Gypsy Major engine is upside down, it makes sense to open the exhaust valve first to drain excess fuel into the exhaust. You will avoid further suck in because the piston will be coming up when the inlet valve is open. Presumably this will also help to blow XS fuel out of the system through the open throttle.
The only other "lighties" I fly have mixture control so you normally do not need to resort to these measures.
P1