taotao 139qmb
 

just put in goofit big bore 100cc kit on my scooter but did not replace the oil pump. scooter starts on occasion but i am getting a vacuum when i unscrew the oil dip stick is this because of the oil pump not being changed out

No, the oil pump will not cause a vacuum in the crankcase.

so what will cause a vacuum on the crankcase

I seriously doubt that you have a vacuum in the crankcase, you most likely have a pressure build up which is normal on these scooters.

But, if I am wrong about there being a vacuum. then that would imply that the vent on the cam cover is sealed as well as the valves and you are trying to pull air into the combustion chamber past the rings from the crankcase side.