50 hp johnson won't run out in water?

Question

I have a 50 hp johnson. It runs fine with the hose hooked up to it and fine in the water in neutral, but when underway, it will only run out to about 1500-2000 rpm and not go any faster. I live around salt water. Is it the impeller. I haven't had it very long, but it ran fine in fresh water during the dealer test. Could it be bad gas. I have put in octane boost and mercury's quickclean. Again, it runs fine in neutral and in gear when hooked up to the water hose.

Answer 1

I know this might sound odd, but it could be that the engine is only running on 1 cylinder. Pull the plugs and inspect them. Look for a wet plug and confirm a spark at that cylinder with a good plug while the plugs are out. Do a compression test on both cylinders.

The engine will run fine out of the water and not loaded because it can accelerate beyond it's normal no-load rpm.

If I'm correct, you have a single carb / 2 cylinder / 50 hp Johnson. I don't suspect carb problems unless you have a 2 carb setup.

If you dare and use caution, you could run the engine and disconnect one spark plug wire at a time to see if the motor runs differently. You may get shocked, so use caution. Good luck.

Answer 2

I had that same engine and here is what the problem with mine was.  

When it was not working it was getting plenty of fuel to build up RPMs so with the muffs on it sounded great.

When it was in the water, it was working and wasnt getting enough fuel and would only go at a fast idle.

The carb was gunked up, I had to take it to a mechanic and he took it off and cleaned it and it ran great.

That was a few years back and I honestly dont recall if I had a single carb or dual. But your engine sounds as if it is doing just exactly what mine did.

Answer 3

You did put oil in the gas??? Did you pull the chock out and forget to push it in>Carbs might need cleaning?? 2 brass plugs in front of carbs remove spray carb cleaner into the jets>

Answer 4

maybe fuel pump or filter