I got a 1976 mercury 20hp model 200 and it was running OK, idled rough but didn't miss at full throttle. now its got to where it just wont start. I try to pull it and it puffs out the exhaust and through carb and it jerks the rope back into the starter almost ripping your fingers off. It starts to crank every now and then but just dies out fast not running for more than 2-3 seconds. The way its jerking the starter rope back acts like it may be out of time but im not sure. I do have a repair manual but i was wanting to get some ideas to where to start looking. I put new plugs they are both firing great on both cylinders but it just wont start. Is it a carb issue making it do this? or a timing? any info be great.