I have an older tecumseh 12 hp. engine on a rider.I is over head valve type. when you engage the starter and it spins, reaching the coil it stops just as though it has to much compression to keep turning over. My first thought was starter, not so. changed three diffrent starters from engines starting and running. NO help. checked valve clearance. No help. Reset air gap. no help, Changed coils,No help. It has good blue spark when plug is pulled and it turns free as you please, install plug and it turns fine till it gets to the coil, then it stops just like the the spark is making it kick itself, and I guess it is, BUT WHY????? I can see no cracks in the fly wheel and every thing else seems to be fine. Oh yes I pulled the fly wheel and checked the Key, no help. I am missing somthing and I cannot find it or see the problem. been playing with motors all my life and this is the first one that has me ready to see what dynamite will do. I am my whits end, Oh yes it has always acted like it had very high compression as long as I have owned it but it always turned over and started, I have had this mower for several years and when I tried to start it from setting over winter this is the results.... HELP!~!!!