75hp Johnson bogs down in river but not land

I bought a boat with a 75 hp Johnson motor and when hooked to a hose it runs fine but when I put it in the river it bogs down. Can anyone help with this?

Do a spark and compression test. Spark needs to jump 7/16 " air gap report back with results. Testing on land means nothing.

Even in gear on a hose it is still neutral.-----The prop does no work spinning in air !!!

And just what year is this 75? There is a whale of a difference.

Is there a way I can check the prop on land

1984 Johnson has 3 carbs.

Spark seems fine but I did notice today that the top 2 carbs are spitting gas through the intake. .. Is that normal

A dynamometer puts a " load " on the engine when on dry land.

fmlylrry45 said:

Is there a way I can check the prop on land

Click to expand...

Put the lower unit in a big barrel then fill with water until the level is above the surface where the lower unit meets the mid section.

The point here is using muffs and a hose puts no backpressure on the exhaust. Without backpressure it only takes a little fuel and spark to make a 2-stroke engine run. By submerging the lower unit in water you're emulating having the motor in the lake and therefore creating a 'real' running condition. You won't want to put it in gear in the barrel, but you can at least start and idle it.

"spark seems fine" doesn't tell us the correct information. You need an air-gap spark tester set to a 7/16" gap to properly test the spark. The reason is similar to running on muffs versus submerged. With ZERO load on the engine the engine can make spark, but under heavy load (acceleration, running WOT, etc) it needs very strong spark to properly fire and make power. So, you can hold a spark plug against the head and see if there is spark on each cylinder. That's fine, but the true test to ensure your spark is strong enough when the engine is under load is to see if it jumps a 7/16" gap. If it does, that means your ignition system is good. If not, then you have some gremlins to track down.

Spitting raw fuel through carburetors is not normal. Well, you can have a little bit of mist in the venturi, but not wet fuel. If that is happening you could have one of several problems: poor ignition on those two cylinders, bad reeds, those carbs are over-fueling.......