I have a 1995 50 hp johnson. It is on a 20' pontoon boat. When I first try to start it fires and quits. I push in the key and give a little throttle and it fires and quits.It takes several attempts and it finally stays running. It runs fine at all speeds and starts fine the rest of the day. I have over 120 compression on both and good spark, new plugs, have cleaned the carbs, replaced fuel bulb. have a built in tank with vro. Any suggestions would be appreciated.