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1978 115 merc runs good for 1 minute

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andynance

New member
"i just got this engine and it

"i just got this engine and it starts and runs good out of the water. i did my compression check and it checked out fine. but i have 2 problems with this boat. #1 it idles fine the water and when i put it into foward and im trimmed all the way dow i will fill and hear a thump thump thump as if it is slipping in and out of gear. its not a grinding sound like when you first put it into gear this sound is when you start to increase speed, it will quite after several thumps. then my boat will plain out and it is fast. now #2 problem it will run fine for about a minute and suddenly loose power and come off of plane. the carbs are rebuilt and i do have new plugs and plug wires ordered, but i dont think that will fix it because ive took this boat out serveral times and on first start up in runs fine. do you think that i could loose fire through my plugs or plug wires after a minute of operation serveral times without any missing or backfireing during that first minute of operation. i can twist the distributer and increase my speed to just plain out but thats it. thank you if you have any suggestions"
 
"#1 sounds like the clutch dog

"#1 sounds like the clutch dog in the lower unit
#2 try pumping the fuel bulb when this happens, make sure the fuel tank vent is open"
 
Those motors do require regula

Those motors do require regular replacement of the waterpump impeller. When was it last changed?Have you checked your fuel supply for any water?
 
"yes i have checked the waterp

"yes i have checked the waterpump impeller and it was fine, i also put in fresh fuel in the tank twice. i will replace plugs and wires on friday and repost if or if not any change."
 
"I had a similar problem with

"I had a similar problem with my 1978 140hp a few years ago. It was the fuel pick up tube touching the bottom of the tank, I have a plastic tank. I also have heard that the fuel lines will leak and suck air if they are old. Changing these things fixed my problem."
 
"Andy,

You say the engine r

"Andy,

You say the engine runs good out of the water and you say when in the water your trimmed all the way in. Could you be getting water into the exhaust and restrictng it too much. Try triming it out some when in the water."
 
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