I'm new here and I have a question about a magneto. I have had trouble starting the engine for years on my 16.5 briggs OHV and at times when I would turn it off I would get a small back-fire. I also have a new mower with a 17.5 Hp, they seem like good engines, my old one is 10 years old and up to spring of this year, it always ran great, then it quit running. The last time I got it started it ran for about 2 mins. and died. I noticed it had a little water in the gas, but anyway, I have had to always roll it to top dead center before starting it and even sometimes I also had to have a charger hooked up as well to get it started. I would get it spining then chock it and it would start. It would always run great after it was started. I had to put on a new starter ring gear and a new gear in the starter drive as well. I decided to fix this one up and build something kinda sorta in a racing tractor. Anyway I tried to start it and again I had to roll it to TDC again. After scratching a hole in my head over what it was doing ( it ran slow back-fired out the exhaust pipe and the carb as well, and the muffler got redhot and while it was running it shot out a nice blue flame while running about six inches from the tip of the exhaust pipe.) I tested the spark and it was orange and sometimes you would see a slight blue spark ( more orange than blue tho ) so now I'm going to put on a magneto, but what is the air-gap to be set at? And do you think I might have another problem with this 16.5 OVH that I have not seen yet? Someone said it might be the cam, but it did run very well when I did get it started. Thanks for your time I just hope someone can shead a little light on this small problem.