Crankshaft shape plays no part in throwing the mixture up the transfer ports. The flats on the counterweight are just a cheap way to machine a crank for reasonable balance. If you look at a Rossi 45 crank though it appears to be a solid disc but it's machined internally for balance and then to make it even better they insert tungsten (from memory) on the side opposite the crank pin. That's the sort of detail you get in a high quality engine