I started my mower 2 days ago, ran it around the yard for a short time then shut it off. I went to re-start it and was just cranking and cranking wouldn't start. I pulled the spark plugs and replaced them, now it only seems to crank over once and then nothing. I pulled the plugs and seem to have spark. I checked the voltage to everything and I have voltage. Pulled the plugs and without the plugs in, the engine turns over fine. Why won't it turn over right with the plugs in?

Could be your valves need adjusting.