1989 40 hp fouling plugs

I have a 89 40hp yamaha that keeps fouling plugs. I purchased this motor early may of 2009 when i ran the motor it wouldnt rev all the way. so I changed the plugs and from there if ran fine. now it is starting to lose RPMS again. what would be some things to check on this motor. it is a jet conversion.

[boats.net]

Compression check, fuel condition(old gas), cooling system(water pump, thermostats), fuel pump and carbs in good working order and if using a good oil.