Be sure you have a high capacity battery. There are batteries that have a 200cca rating or below. I think this motor needs at least 350cca. I went through the same situation a while back until someone told me that this engine bore was about the equivalent of a 327 chevy and required about the same battery power to turn over.I have an old Murray mower that I am repairing. It has the 14.5 hp single cylinder over head valve engine. It is very hard for the starter to turn the motor over unless the battery is fully charged. It blows a little bit of oil up the dipstick. It has a lot of compression, but it does not smoke. Do you guys think this is a head gasket failure?