Modifications may be required when installing a Parmakit with a Doppler crank (specifically based on the installation on a 1977 Motobecance Mobylette AV7 50 engine).
Issues and Solutions
Exhaust Port Clearance
At BDC the piston did not completely clear the exhaust port of the cylinder.
Use spacers to increase the distance between the cylinder and the engine case.
Make sure of the following:
A. The Piston dips just below the exhaust port at BDC.
B. The exhaust port is completely covered at TDC.
C. The piston completely clears intake port at TDC.
D. The piston completely covers the intake port at BDC.
Reduce Squish Clearence
Now that the cylinder is further from the engine case, the volume of the expansion chamber at TDC is significantly greater than it was before. If you're using the stock AV7 head you don't have much to worry about. However, if you bought the high compression Parmakit head (or possibly some other high compression head), you squish clearence is greater than it should be and it must be reduced to prevent detonation.
Detonation is the fuel mixture being ignited by the heat and pressure in the combustion chamber, usually at the outer edge of the piston. When that combustion "event" collides with the ignited fuel from the sparkplug you get the "marbles in a can sound" from the motor. Detonation is very hard on the bearings as the loads on the bearings are greatly increased. Also the action of the collision of flame fronts, strips off the thin boundary layer of mixture (about .020" thick) that actually protects the metal parts from the intense heat. Signs of detonation are the aluminum parts are pitted, and usually metal flakes on the spark plug. In bad cases a hole will actually be burned through the top of the piston. 
The top of the cylinder must be sanded or milled down such that the squish clearence is about .040".  If the squish clearence is too large you risk detonation. If it's too small your piston will smack the head at or before TDC.