Re: Mid 50’s Johnson 25HP runs like crap. Advice needed

[Don]

It only takes a minute or so of running the impeller dry to damage it, so please don’t run it outside the tank.
Is it not convenient to run the engine on a boat? If not, we could try another approach. Start the engine in the tank and let it warm up so it will be easier to start once it is out of the tank. Next, remove the gearcase so the engine can be run on a sturdy stand with no water. Rig up some sort of hose connection to the water tube inside the exhaust housing….Start the engine with no water running through it, see how it runs, the turn on the water supply to see if its running quality changes. Have a look at the plugs after running with no water, then compare them to how they look after turning on the water supply.
Again, it will be tough to compare running quality between running in the tank and running out of the tank with the gearcase removed….Running it out of the tank with no gearcase removes all exhaust back pressure and the load of turning gearcase components….So, it would be easy to conclude that the "engine runs better" with the gearcase removed, and head down the wrong path.