Crankcase stroke 57mm conversion to 60 mm conversion, Polini 177 cc

Dear,

Changed my standard crankshaft into a 60 mm long stroke for reed valve.

As indicated by sip, I compensated the 3mm extra stroke with one 1,5 mm gasket between Cylinder & cylinderhead.

Tried to start the engine, but no vacuum present to suck up the mixture of air & petrol....(no movement of the reed valve membrane)

Changed the setup...placed a 1,5mm base gasket (between engine case & cylinder) 

Some vacuum and movement in the reedvalve noticed now...but not enough I think to open the membran of the reedvalve wide enough.

Do i have to modify my piston ?