Mercury 90 FOURSTROKE Carburetor Engine

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bmat5
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Mercury 90 FOURSTROKE Carburetor Engine

Postby bmat5 » Mon Feb 08, 2021 9:19 pm

I'm about to purchase a 2005 Montauk with Mercury 90 FOURSTROKE Saltwater engine. I realize fuel injection is preferred, but I'm okay with carburetors and like the boat. It's a first boat, and I intend to use it on the bay for fishing, crabbing and reaching the sand bars. I haven't found too much information on the Saltwater engine, other than apparently it included more stainless components. The present owner had the carburetors rebuilt a few years back, and claims to have never run anything other than ethanol-free fuel since beginning ownership. I'm looking for input on this engine and fuel delivery system. Thanks.

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Phil T
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby Phil T » Tue Feb 09, 2021 9:54 am

I would not be too concerned about [the Mercury 90 FOURSTROKE carburetor engine].

Prior to purchasing I would do a compression check and visually inspect under the cover to assess the general condition.

Running ethanol-blended fuel is acceptable on carburetor engine. To avoid phase separation add the manufacturer-specified amount of stabilizer product for every gallon of gasoline to the tank.

If you are in a cold climate, add double dose for the winter storage period.

ASIDE: I ran an older two-stroke-power-cycle engine in Maine on my classic Montauk using ethanol-blended fuel for many years and never had a fuel-related problem by following the above guidance.
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jimh
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby jimh » Tue Feb 09, 2021 1:15 pm

I infer that the Mercury 90 FOURSTROKE carburetor engine is probably also a c.2005 vintage and came with the boat as the original engine.

There is a good probability that this engine is actually a Yamaha power head engine. Around c.2005 Brunswick had not developed its own four-stroke-power-cycle engines, and Brunswick entered into an agreement with Yamaha to buy engines from Yamaha. Brunswick then repainted and rebadged these engine under their Mercury brand. Most of these engines used carburetors. Only a few Mercury-badged Yamaha engines were produced with electronic fuel induction (EFI).

Considering the overall general trend for use of boats to be at sea level or not much higher than perhaps 600-feet above sea level and to be in temperatures ranging between 60 to 90-degrees, there was and remains really nothing wrong with carburetors in those conditions, and carburetors were used for decades.

There were reports of problems with the carburetors on these Mercury-made-by-Yamaha engines. The problems seemed to be associated with the very low rate of fuel flow at low engine speeds (like at idle). The carburetors were reported to become clogged. I believe that these were real problems for some users. You can find many prior discussions about these problems in the archives. For example, here are several old discussions on these engines:

Mercury 90-HP FOURSTROKE: Carburetor Problems
http://continuouswave.com/ubb/Forum3/HTML/009936.html

Mercury 90-HP FOURSTROKE Carburetor Repair Bill
http://continuouswave.com/ubb/Forum1/HTML/015765.html

Mercury 90-HP Four-stroke Unreliable; Carburetor Problems
http://continuouswave.com/ubb/Forum3/HTML/009067.html

2004 170 Montauk 4 stroke carburetor problems
http://continuouswave.com/ubb/Forum8/HTML/002726.html

If you read these threads you will find first-hand accounts from people who owned the engine. You will also find comments from people who did not own the engine but were enormous fans of Mercury-brand engines who attempt to lay all the blame onto Yamaha. You will find comments from very experienced mechanics who owned these engines. You will find comments from people who were not mechanics, who did not own the engine, and who were big fans of Mercury. You should consider which sort of comments will have the most information value.

As for the care and feeding of these c.2005 Mercury 90 FOURSTROKE engines, I suggest the following:

--do not intentionally try to drain all the fuel from the carburetors between uses
--stow the engine in a vertical not horizontal or tilted-up position to minimize fuel draining out of the carburetors
--always treat the gasoline fuel with a cleaner additive; Mercury makes several such fuel treatment products, as does Yamaha, as does Evinrude, and as does many aftermarket providers
--try to use high-detergent gasoline from major fuel providers, and use ethanol-free fuel if available in preference to blended fuel
--use a 10-micron fuel-water separating filter upstream of the engine to ensure that no water or particulate debris gets to the carburetors
--avoid letting the engine sit unused in warm temperature and strong sunlight outdoors for weeks between uses as those conditions accelerate fuel evaporation
--before engine start be sure to prime the fuel system with the primer bulb until the primer bulb becomes firm
--before engine start carefully read and follow the engine's exact starting procedure; every engine has its own starting procedure with its own peculiarities, and do not assume you already know how to start engine and use some different procedure.

jimh
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby jimh » Tue Feb 09, 2021 1:57 pm

A good description of the fuel system is given in the first of the four articles I gave links for above by SOSMERC. SOSMERC is an expert Mercury engine mechanic.

bmat5
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby bmat5 » Tue Feb 09, 2021 8:22 pm

Much appreciated Jim!

jimh
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby jimh » Wed Feb 10, 2021 5:55 am

Another aspect of the Yamaha-made Mercury-branded 90 FOURSTROKE carburetor engine that is a bit unusual is the four carburetors, one for each cylinder on a four-cylinder engine. Having four separate carburetors means four linkages to keep them synchronized. In the mentioned articles (above) there are comments about needing to use specialized test equipment to synchronize the carburetors. In 2005 multi-carburetor four-stroke-power-cycle outboard engines were becoming rare, and in 2021 in new engines you don't see much use of multi-carburetor engines at all.

I think outboard engine service technicians who are experts in carburetor set-up are probably getting rare, too. For that reason, these four-carburetor engines probably will be best serviced by an older and experienced technician who has worked on lots of multi-carburetor engines. A newer outboard engine tech who has only been working on engine with electronic fuel induction with computer controls probably won't be the best guy to work on these engines.

ScooterII
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby ScooterII » Wed Feb 10, 2021 6:55 pm

I had a 2003 Mercury on a 170 Montauk [presumed to be a Yamaha-made 90-HP with carburetors] and sold it in January 2021.

The [2003 Mercury engine presumed to be a Yamaha-made 90-HP with carburetors] needed a $1,000 repair to a corroded exhaust pipe in 2015. The [2003 Mercury engine presumed to be a Yamaha-made 90-HP with carburetors] ran great afterwards.

bmat5
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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby bmat5 » Wed Feb 10, 2021 7:07 pm

jimh wrote:A newer outboard engine tech who has only been working on engine with electronic fuel induction with computer controls probably won't be the best guy to work on these engines.


Point noted. I've messed around with carburetor set ups, yet never with four carburetors that needed to be in sync. [Getting four carburetors in synchronization] sounds like a bit of a challenge for techs at the time. I'd imagine finding the right tech now would even be more difficult.

Are any readers running these engines today?

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Re: Mercury 90 FOURSTROKE Carburetor Engine

Postby Jefecinco » Thu Feb 11, 2021 9:48 am

In the 1960s I used a "Uni-Syn" tool to synchronize multiple carbureted Porsche engines. As I recall the tool was not expensive in Germany where I was living at the time. Amazon sells them for about $23. I must have paid for that tool a hundred times over synchronizing carburetors.

Disclaimer: I know nothing about the engine under discussion but suspect a Uni-Syn could be useful for the job.
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