I have a ferris pro cut z model pcz25k engine model cv25s, my question, why wont it stay running? was mowing it stopped ,like it ran out of gas, i put gas in it tried to restart it , will only run in the start position. this is what i changed - fuel filter air filter carburetor fuel pump ignition switch, gets spark in start position checked still only runs in the start position, will only stay running in run position for no more than 2 seconds