Moped Army

So, I bought this 1993 Golden Bullet non running for a $100, boohaha! Cleaned out fuel line carb. and so on. Started and run fine after a bit of tinkering.
So, here the deal... It had a standard setup. Dellotro SHA 14/12P carb and intake box/silencer and standard exhaust. It also had a #51 jet. (witch seems small and I don't know if it is the right size.)
The bike ran fine up to 28 MPH and then started, what i now know as ''four-stroking''.
I took out the air box and drilled a few hole in it and put a #52 jet in. Then it ran 20 MPH and started ''four-stroking'' again.
Can someone tell me if this is because of to much gas or air? And what size jet should i use or what should i do to the air intake?
thank you.