Did you check if your fuel tank is not leaking at the seam around the cap? I had that problem with one of my YS installations after about a couple of hundred flights. The leak allowed enough pressure for the plane to fly, till it got worse. A pressure check on the system revealed a slow leak, much more leakage under pressure. Also check the regulator as indicated by others here. I have 4Ls and they all run great. I run YS20/20 or CoolPower Pro Pattern 20 when I cannot get hold of YS20/20.

PineBaron