Thread: Merc 50HP 2-Stroke Running Rough

Originally Posted by MNCrapps

Have a 2006 Merc 50 HP 2-stroke that came with a used Tracker PT165 that I bought last fall. The motor ran rough from day 1. I thought I figured out my problem when I found that the inside lining of the hose was disintegrating. I replaced the fuel line with automotive fuel hose, re-built the fuel pump, and replaced the fuel filter. All along this path, I found pieces of the old fuel hose lining. While the motor does run better, it still doesn’t seem like it runs right to me. It runs the roughest when I trim the motor up at idle. The more I trim it up, the rougher it runs and dies most of the time when I trim it way up as I approach a boat launch. I’m thinking I may rebuild the carbs next. Thoughts? Could pieces of the old fuel hose lining be in the carbs even though they are on the other side of the filter? Why does it run the roughest when trimmed up?


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how gallon fuel tank do you have ???? take a put a can of Seafoam in it and run like you normally do, but when you come back to the boat ramp run it wide open for a while then back off to normal !!! load it up and see if the next time out if run better !!! and do same procedure when come back in !!! that Seafoam will clear up some stuff for sure !!! I put in my fuel every other fill up just to keep my motor clean been doing that for years !!!!

The boat has a 13 gallon built in tank. I did run 2 cans of sea foam through it before I discovered the fuel hose issue but not since rebuilding the fuel pump. I will try your suggestion. I should also mention that i have an extremely difficult time adding gas to the tank. The pump kicks off on me repeatedly and gas spits back. I think a trap has formed in the vent line.


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It's probably running rough when you trim up because the carb bowls are not as full as they should be,when the gas tilts in the carb bowl motor having problem getting gas.

I have had several boats and trucks that wouldn't take gas right because of the way the fill tube and vent were piped and the size of the fill tube. I bought a new Dodge ram truck in 2008 and they recalled the fuel tank for that very reason. You might check to make sure the vent is open but could be just bad design on fill system.

If you had any fuel breakdown you may just have some gummy stuff somewhere. I am thinking sounds like not enough fuel getting into carbs at low speed ( fuel pump is pulsing slower pressure falling too low to keep carbs full ?) .

If it where in carb jets or passages should idle rough all the time. Sometimes more than one problem..two or three small things adding up to create one problem you experience.

I'm just a hack offering something so keep that in mind.

a rough idle usually means a carb rebuild is in order. I think doggone is correct, as you trim it up, the carb bowl flow changes, and if you've got buildup or old hose pieces floating in there, you're going to experience a rougher idle. carb rebuilds are relatively easy and cheap, I'd start there before investing anywhere else.

Hello: After you fix all your problems I would add a fuel filter water separator in the fuel line. It's about the size of an oil filter. I replace it every year. I have an 05 Merc that had problems. I cleaned out the whole fuel system and then added the fuel filter water separator . Never had any issues since. Still add can of Seafoam a couple times a year and right before storing for winter. Good Luck . O - R

Originally Posted by outdoorsron

Hello: After you fix all your problems I would add a fuel filter water separator in the fuel line. It's about the size of an oil filter. I replace it every year. I have an 05 Merc that had problems. I cleaned out the whole fuel system and then added the fuel filter water separator . Never had any issues since. Still add can of Seafoam a couple times a year and right before storing for winter. Good Luck . O - R

Thanks for the advice. Where did you mount the fuel filter/separator?


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Originally Posted by hc803

a rough idle usually means a carb rebuild is in order. I think doggone is correct, as you trim it up, the carb bowl flow changes, and if you've got buildup or old hose pieces floating in there, you're going to experience a rougher idle. carb rebuilds are relatively easy and cheap, I'd start there before investing anywhere else.

Thanks for the advice. I’m pretty sure I need to rebuild the carbs.


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One other thing I would check is the fuel fitting at the motor
It may have a bad o-ring that leaks air when tilted
But I would also rebuild the carbs



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