I have a 67 evinrude 40 hp selectric shift. I can get it started but it only runs a few minutes and dies. in that time it will start to backfire a little bit. any suggestions. I have replaced the points, coils, capacitors. I am also having problems with the water pump pumping water out of the water exhaust. I have replaced the waterpump and housing and also pulled the thermastat. help please