Driving to work the other day and my '78 40 died on me. Tried to start it again, would seem to fire as long as the starter is engaged, but won't quite catch (so would just die off as the key was let off). Has even spark. Changed the carb with a working spare I had in the garage. Neither helped.. checked for fuel pump flow, seems to be pumping out a fair bit of fuel when I crank it (and the fitting is off the carb..) Any ideas???