- ignition is different ( 600p has 3 advance stages and the 620p has 4 advance stages at different ranges of each other)
- carb is different ( Walbro HDA 268 for the 600 and Walbro HDA 316 for the 620)
- cylinder timing and porting is different
- compression ratio is a little less on the 600 than on the 620,...
stock WOT on a 600p is 13.000rpm
stock WOT on a 620p is 13.400rpm
...