getting slower with cross exhaust?

Got a 50ccm luna with 75er Casa Kit, still running the original carb (14er), 60 main jet. Today I put a vega up+over exhaust on it, which makes it drive slower than before....!?
Acually I thougt that I should even gain instead of losing power.
Would open the main jet help to keep driving faster?

Im no expert but I would of thought that you should be looking for a larger main jet and maybe even carb as soon as you increase the cc of the scoot.

Is it defo a Vega up and over and not a 50 one from the Swedish 50's?

its an italian 75er exhaust. the only thing that worries me is that the luna was running perfectly with the original exhaust. will try a larger main jet and look forward!