1) check that your hop up rubber is set up correct (get a gun doc to show you how)
2) your new ported cylinder may not be the right one for the barrel length. - Check with water to see if when you fill the barrel up with water, and pour it into the cylinder, the water should reach near where the port is. if its to low or over the port you can get air problems when they exhaust the barrel.
3) check for piston compression. - its a common problem that the o ring on the piston head is not making contact with the cylinder wall. (cover the cylidner head nozzle with a finger and push your piston up and down. you should feel pressure build up. if not you have a contact problem and needs to be re-greased or the oring changed)
Those are the most common problems, if those dont fix it, we will have to go into complex solutions
