Hello, <br /><br />A friend gave me his 1979 20 hp Envirude which had an oversized spark plug hole and stripped treads. Originally the engine would start without problems and then would die as a result of water stopping circulating and overheating.<br /><br />I just changed the cylinder head, thermostat and gaskets. Put new champion plugs (L77J4C)gapped @ .040. Added fresh gas with 50:1 oil ratio and tried to start the thing. Well, after few pulls this huge bang came out of the exhaust, almost sending me to the floor. I tried cranking few more times but more gun shot sound alike came out of the exhaust. The engine sounded like it would almost start but nothing at the end. <br /><br />Is this what you call detonation? What is the difference between detonation and backfiring?<br /><br />I noticed that at TDC the flywheel was indicating about 24 degrees instead of 34 degrees. Could my starting problem be caused by the timming being 10 degrees off? If so, why was the engine starting before I changed the head, gaskets, thermostat and plugs and now it won't start?<br /><br />Thanks again for your help.<br /><br />Veronica
