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a 73 Johnson 50hp to idle in and out of gear great when the boat is out of water but when you put the boat in the water kick it in gear to idle out it will go dead. It will idle in neutral ok when in water but kick that baby in gear and putt, putt, dead, restart.
There's back pressure on the exhaust when it's in the water, so problems show up there when they don't on a hose. Had the exact same thing happen once and it was caused by old gas.
Low compression will do that as well.
i bought a boat last spring and it had a 60 merc on it that had been worked on and it did the same thing and i found that the fuel line was being pinched shut by the hood sometimes not all the time but when i would get it home and check everything some times i guess it would get pinched took me awhile to find it but i shortened the hose and works fine now . might even want to check your fuel filter if you haven't already .
scott
Your carbs need to be adjusted or even rebuilt. Normally final adjustments on carbs is done in the water and in gear.
I had the same problem a few years ago. I had to have my carbs rebuilt.
I have a 74 Johnson 55hp doing the same thing. I enededup having to rebuild the whole engine. Cost about $1700 and some think its not worth it, but then how are you going to replace it for less.