On the pro18 it is actually quite difficult to prime, all the small block TT engines seem to be. Make sure you manually open the carb with the power off, prime then once it reaches the carb, flick the lid of the tank twice, then again half a flick. I found this the best method for this engine. By flicking I mean hold/pinch the exhaust breather and open and release the flap on the tank making a kind of flick. This action makes the fuel in effect, pump into the carb. Its deceptive on the .18 as you need more 'flick' than you think you might. 2 1/2 is good for this engine