I have a 1969 merc 6hp that seems to have a problem with the fuel pump. It runs if i keep pumping the bulb, but shuts down slowly if i stop pumping it. At the end of last season i got a new fuel pump kit without the check valves. It seemed to help for a while, but by the end of the day it had the same problem. A new fuel pump kit with check valves is almost $50 compared to $5 without, so i don't want to spend the money if something else could be the problem. Any thoughts?