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Author Topic:   Mercury 50-HP Runs Poorly, Gurgles, Grumbles, Stalls
cleep1700 posted 05-14-2015 07:53 AM ET (US)   Profile for cleep1700   Send Email to cleep1700  
Last Fall [I] bought a [1978 boat] that came with a Mercury 50-HP of the same vintage. [I] had Bass Pro go through the engine, and they cleaned up the carburetors among other things. Recently [I] took out [the boat] for its first run and noticed that the engine seems to lag when halfway to full throttle. A couple of times it stalled. I fired back up [the Mercury 50-HP] and then slowly moved the throttle forward. [When the throttle] reached the halfway point, [the engine would] gurgle and grumble, and then it would slowly recover and sound as if it were running on all cylinders. This happened intermittently during a two-hour run.

Is it normal for an old engine [to lag at half throttle, to gurgle and grumble, and do this intermittently after having been serviced]?

I took it back to Bass Pro. At the end of the day [Bass Pro] said [the engine] sounded like the injectors might be plugged up and an additive to the gas mixture is a possible solution.--Craig in windy Kansas

tedious posted 05-14-2015 08:17 AM ET (US)     Profile for tedious  Send Email to tedious     
No, it's not normal, and it's very likely a carburetor problem, where the carburetor opening is not properly synchronized with the spark advance. Any mechanic familiar with that motor should be able to fix it pretty quickly.

Sounds like it's time to find another mechanic.


jimh posted 05-14-2015 08:56 AM ET (US)     Profile for jimh  Send Email to jimh     
CLEEP writes:

[I] had Bass Pro go through the engine, and they cleaned up the carburetors among other things.

Then, when the engine is not running well, and is returned to Bass Pro:

[Bass Pro] said [the engine] sounded like the injectors might be plugged up

I see two problems:

--how does the engine have both a carburetor fuel system and an injector fuel system simultaneously?

--how does a 1978-vintage Mercury have fuel injection? I don't believe Mercury made engines with even the most simple fuel injection system until the late 1980's; your engine seems like it is ten years too old to have fuel injection.

Also, since these problems with poor running seem to have manifested themselves after the boat's "first run"--here I assume you mean after a winter lay-up period or a period of long storage--it seems likely that the cause could be due to evaporation of fuel in the carburetors which has left behind some residue.

If an engine has a clean fuel system, the use of fuel additives which are detergents or stronger solvents is a good way to maintain the fuel system. Sometimes an engine with debris, dirt, residue, clogs and other problems in its fuel system may be improved by operating the engine on fuel that has been doctored with added solvents or detergents, but that method may not always provide a remedy to every problem. If the engine has already received service from professionals to "clean up the carburetors" then it seems odd that the same service professionals would later recommend the use of fuel additives as a method to remedy problems in the fuel system. If the carburetors were properly cleaned, they shouldn't be causing problems and you shouldn't have to use fuel additives as a remedy for those problems caused by the carburetors.

Binkster posted 05-14-2015 12:37 PM ET (US)     Profile for Binkster  Send Email to Binkster     
It is next to impossible to find a modern dealership that has the tools, knowledge, manuals or inclination to work on an outboard that is older that the working personal. Find an independent old time mechanic to work on you old Mercury or join the Antique Outboard Motor Club in your area. The members will help you and teach you to do the work. They love old Mercury engines, too. Here is a list of the chapters.


jimh posted 05-15-2015 11:24 AM ET (US)     Profile for jimh  Send Email to jimh     
Rich--you're right: a c.1978 outboard engine is 37-years-old, and that is probably older than most of the people working as marine technicians these days.
cleep1700 posted 05-18-2015 04:33 PM ET (US)     Profile for cleep1700  Send Email to cleep1700     
Thanks. The problem was solved by finding an older fella who knew the engine well and had been working on Mercury outboard engines for over 40 years. This forum is so essential for the newbies like I am. Thanks again--Craig in windy Kansas off to Colorado to fish from my Whaler and very excited about it.
tedious posted 05-18-2015 05:36 PM ET (US)     Profile for tedious  Send Email to tedious     
Just the thing. Glad you got it going--Tim
weekendwarrior posted 05-19-2015 01:17 PM ET (US)     Profile for weekendwarrior  Send Email to weekendwarrior     
So what was [the cause of the problems] with [the 37-year old Mercury outboard engine of 50-HP]?
jimh posted 05-26-2015 12:22 PM ET (US)     Profile for jimh  Send Email to jimh     
I sent an email to the original poster to solicit his further participation. [He replied he is on vacation and no computer but will follow-up later.]
outragesteve posted 05-26-2015 06:10 PM ET (US)     Profile for outragesteve  Send Email to outragesteve     
Possibly bad stator? High speed circuit failure. Just a guess.
cleep1700 posted 05-27-2015 04:02 PM ET (US)     Profile for cleep1700  Send Email to cleep1700     
Thanks for all of the input and suggestions. The upshot of the entire episode was that I was dealing with contaminated fuel. As suggested, I took the motor into a fella who was older than the engine who knew them well. He checked everything out and determined that the fuel was the problem; we dumped it; put in a fresh batch of non-ethanol pure gas 87 octane with some Mercury Quickcleen additive; and it runs like a top. Moral of the story for all of the rest of us newbies: check the fuel first and go from there. It also helps to have someone who knows your engine. I thought I was in good hands at Bass Pro, but the techs knew nothing about this motor.
I'm out in Colorado at the moment fishing some high altitude problems at all.

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