All I did was pull the engine out then reinstall, after the reinstall I could'nt start it. I never even touched the regulator till after I started having this problem. I double checked the fuel & pressure lines & they're hooked up as should be, the check valve is in the pressure line so as to hold the tank pressurized. The last time I ran this engine(couple weeks ago) it ran great, I shut it down after a ten minute flight, drained the tank & ran it dry, did'nt notice anything unusual.