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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15875
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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My 1998 Mercury 50HP 4 stroke is stuttering when opened up.

Customer Question

My 1998 Mercury 50HP 4 stroke is stuttering when opened up. It runs fine at low 2000 RPMs. I think the thermostat needs replacing. It has happened before years ago. Where is the thermostat located or what should I look for. Water stream is good and not hot.
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.
Good morning, my name is ***** ***** am I speaking with?All Mercury engines are cataloged by serial numbers. Can I that number so I can run it and see what you have.
Customer: replied 2 years ago.
I'm Scott,Looks like 0G620480 is the Serial Number. I did put in a new thermostat and didnt fix the problem. Stutters or runs rough above 2200 RPMs New plugs about 3 hr of run time. No water in the fuel. Any ideas?
Expert:  Jason replied 2 years ago.
Hi Scott. Out of curiosity has this ever been in for dealer level service? (If yes, when was the last time). And any idea of the total hours on the engine?
Customer: replied 2 years ago.
I head gasket issues about 3 years ago but that was fixed that is why I checked the oil to see if water was in there. Carbs were gone thru about 5 years ago. I use Seafoam on occasion. I put around 15-20 Hrs per year at 90% at low RPM while Pontooning
Expert:  Jason replied 2 years ago.
The reason I asked on dealer service is because the carbs on these engines do go out of balance, and they really need to be adjusted every 150 to 200 hours. By adjusting them what you are doing (or the mechanic) is balancing the amount of vacuum in each intake manifold runner. When vacuum is uneven in cylinders what happens is each cylinder does not get an even amount of fuel. Some get more, some get less, and this causes the engine as a whole to fire unevenly. Giving you the rough running. The tool I use for balancing carbs is called a Morgan Carb Tune. If you brought this engine to me what I would do is hook up my carb tune and check the vacuum on each intake runner. If they are out of whack I would re-adjust them until they were set to factory specs, and re-run the boat. If it's better, that is all the problem was, was the carb's needed some normal adjustment. If it still runs rough, then chances are 1 or more of the carbs has a little bit of crap in it, and then I would pull them, clean them, reinstall and re-sync. Does that all make sense to you? Do you you have any further questions on this issue?
Expert:  Jason replied 2 years ago.
Good morning. I see that no rating has been put through yet so I am just checking in. Did you have any further questions on the issue?

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