I tried to adjust my valves, following the directions in the manual word for word. I was very careful and took my time. When I was done, it started up and ran just fine. I proudly watched a smooth-running engine for about 2-3 minutes before turning it off. But when I tried to restart it later, it would not start. The starter just keeps turning it over without results. The sparkplugs are like new and correctly gapped. The distributor cap and rotor are same. What's going on?