Give us some info first. What boat is it on? How well has it driven in the past? What rpm did it get when it ran well? Do you have a fuel filter in the system? Have you tried to replace it. Have you replaced spark plug wires, spark plugs? Have you checked your lower end oil? I would dump it and check it for metal and water, sometimes a lower end problem looks like an engine issue. Do you have to run ethanol fuel? If so I would obtain some fuel hose,a small marine gas tank, fill fresh fuel and hook the new fuel line directly to the engine. Just temporarily set it on the floor of the boat and go for a test run. If it work okay chances are you will need to pump out all of the junk fuel and replace it with fresh. Next replace the entire fuel lines from tank to engine. Ethanol is tearing up fuel systems. And even if you do not use ethanol fuel you may have gotten some trash fuel from somewhere. Another thing to consider doing is running a can of Sea Foam in that small tank. You could have dirty carbs because I think that is a carbed motor. I have very little Johnson experience but if they have a diaphragm fuel lift pump it could also need to be replaced. Most of my experience over 40 years has been with Mercs sorry. Give us some answers and we will try and get a little more specific. I would start by replacing the maintenance items I mentioned. Sometimes we forget to do the simple things, I hate wasting money but to replace old spark plugs, wires, fuel filters, lower end oil etc is hardly a waste. It is improvements in the engines running condition and a simple way to check the items off of the troubleshooting tree. Good luck. Let us know how it progresses. It still could be more involved but I would check off the easy stuff first.