A lot of good advice and none of it wrong. But I think some of the simplest things that can cause major headaches have been neglected.
I think you will find that the motor specs calls for 91 octane. Using regular can cause startability and low speed problems. Change the thermostats. If they are stuck open the motor is running too cool, especially right now, and the motor will have incomplete combustion fouling the plugs causing startability and low speed problems. My 95 Mercs have diaphragm fuel pumps. I don't know about a Johnson but if it does, rebuild it. Stretched out diaphragms once again cause startability and low speed problems. Remove the plugs and spray with DeepCreep (aersol SeaFoam) let sit overnight and take it and a spare set of plugs to the nearest lake the next day. Keep it on the trailer until you get it started and it is running smooth. It will smoke like you've never seen before. Pull it out of the water and change the plugs. Change all filters, check to see if there is one under the engine cowling. If you have a fuel/water separator without a clear bowl, bite the bullet and upgrade to that. totally eliminates the "Do I have water in my fuel", question.
A good service shop will include most of these repairs (actually preventative maintenance) in some fashion. A lot of engine problems masquerade as carb problems but sometimes the real fix in addition to a carb job is ensuring correct fuel delivery and complete combustion in the cylinders.