1999 50 HP $ Stroke problem

[Yakmaster]

I have a 1999 50hp 4 stroke with an ongoing problem that I need help with please
Fishing yesterday motor running fine idling and under load at low speed as soon as the motor was loaded up it felt like a cylinder was constantly missing (only under load). Removed and checked the plugs and appeared no. 3 plug was fouled and not firing, changed plug no change to motor performance. more checking at home today fuel supply appears ok, removed plugs connected to coils and fired over, checked all sparks looks ok.
Previously similar issue not resolved checked compression, replaced head gasket, removed carbs cleaned replaced and adjusted, new thermostat - all done by yamaha certified mechanic, replaced computer and temp sensor. first run yesterday -> back to the start?? help

Frustrated

 

[achesley]

I had a like problem with a Nissan. After warm, would go to 2 cylinders slowing down but run fine at high rpms. Dealer never found the problem. I finally went at it even though still under warranty. Pulled on plugs wires pretty hard at the boot fitting over the plugs. One pulled loose. Seen where it was a little burnt. Cut a bit off and attached it right. Run great after that.

 

[qhn]

Focus on the mechanical pump and the electric pump. Need to check fuel pressure, clean the vst filter. But my bet is your mechanical pump failure. If you have strong, consistent spark and get a miss out of it. Fuel it is.