My 1970 115 Johnson was running real sluggish and rough at higher speeds, but fine at low speeds and started good. I put new plugs in, cleaned the high speed jets. Now it won't even start. I have new plugs, good compression, hot spark, and fuel going into carbs. When I try to start it gives me a little puff of white smoke from the exhaust now and then but doesnt even try to start. Any suggestions? Could there be a timing problem with the spark? Please help.