I suspect this is a result of simple evaporation. Coupled with heat cycles. As the Can heats it would force gas out through the tab. I'm willing to bet the concentration of the syrup would be higher and replacing that 150 ml with water would bring it close to the original formula.
Could Be...
I'm thinking a flywheel with a large reduction ratio would help maintain the available energy from the mousetrap spring Without inducing massive wheelspin.
Might kill your top speed but should go for awhile.