RE: Problems with short run time?
There are a couple of things to look at. To start with, check your gear mesh (pinion/spur). If it is too tight, the motor will be working too hard, increasing motor heat and reducing run-time. If your gear mesh is ok, try going to a smaller pinion. Your top speed will be reduced, acceleration increased, and motor load reduced (increasing run-time).
One more thing: How much run-time were you getting with the factory 19T and with the 27T? What battery (capacity) are you using? What charger do you have?