I recently rebuilt two 2 stroke scooter engines (Kymco Super 9 & TNG Venice LX), and I always look at why the engine failed to protect my work. Like many of you, I started the engines with premixed fuel and proceeded to bleed the oil pump and adjust its flow.
Neither scooter ever bleed out the pump, so I took a look. The Kymco pump is easy to service, and I disaasembled it, cleaned, checked the gear mating, relubed, reassembled and reinstalled. Works fine.
The TNG oil pump is behind the stator, and a little more work. Pulled the pump, and it was rotating easily, so I bleed it with a drill. Reinstalled it, but it still would not pump oil. On closer inspection, the drive gear attached to the crank was not mating with the oil pump gear. Pulled the drive gear and found a concave wear pattern on the (nylon / plastic) gear.
The wear pattern is really hard to tell even with the gear out. I would suggest if you have one of these scooters that you replace the gear as a normal maintenance item and save a lot of money from engine damage.
here is a picture of the gear:
