1987 Mercury 90-HP Two-stroke-power-cycle Engine Won't Run Under Load, Won't Restart

[Avagab0nd]

I have a 1987 Mercury 90-HP two-stroke-power-cycle outboard engine on my recently-purchased 1977 Montauk 17.

As an initial test of the engine, I ran the engine on a hose adaptor on the trailer.

In September 2023 I got the boat on the water on a lake. I went once around the lake at the slowest possible boat speed.

On the second lap around the lake, I increased the engine speed. The bow of the MONTAUK 17 lifted up. The engine shut off. The engine would not re-start.

Later I again tested the engine on the hose adaptor on the trailer. The engine did not shut off when the throttle was increased.

Advice from friends makes me think the cause of the engine shut off and failure to re-start may be due to the carburetors.

I plan to install a new fuel-water separating filter and a new in-line fuel filter.

I have been told that buying replacement carburetors will be less costly than to rebuild the original carburetors.

Q1: where can a Mercury carburetor designation 1090727 be purchased?

Q2: will buying rebuilt carburetors be less expensive than rebuilding the original carburetors?

Give me advice on how to replace the 1987 Mercury 90-HP two-stroke-power-cycle engine's ignition system. The original ignition system is loose and is damaged.

--Chris

[jimh]

Later I again tested the engine on the hose adaptor on the trailer. The engine did not shut off when the throttle was increased.

Testing engines on a hose adaptor with no load on the engine is not a useful test. Without a load, the engine will accelerate easily to probably the maximum allowed engine speed--or higher--even when not all cylinders have combustion. Do not advance the throttle of an engine when it has no load, as the outcome is not useful and you could easily over-rev the engine.

Advice from friends makes me think the cause of the engine shut off and failure to re-start may be due to the carburetors.

A problem in the carburetors could cause the engine to run poorly, but many other elements of the engine operation that are not in good repair can also cause the engine to run poorly.

I plan to install a new fuel-water separating filter and a new in-line fuel filter.

These are inexpensive parts that often need periodic replacement. Unless the last person to install these filters marked the date on the filter, you don't know how long the filters have been in service. Generally a gasoline filter only needs replacement when the filter media become blocked by debris. If good quality fuel has been used, there is usually no debris or water in the fuel, so these filters can last a long time.

Q2: will buying rebuilt carburetors be less expensive than rebuilding the original carburetors?

Unless there is some enormous premium cost for the necessary carburetor re-build kit, I cannot imagine that an OEM carburetor rebuilt kit would be more expensive than buying a new OEM replacement carburetor. That makes no sense. If you hire a professional mechanic to rebuild the carburetors, there will be a labor cost, perhaps more labor involved that just pulling out the old carburetors and replacing them with new ones. I would not expect that a mechanic's labor charge to rebuilt a carburetor would be such a cost that buying a new carburetor would be cheaper. Also, you could rebuilt the carburetors yourself, which would certainly be much cheaper than buying new carburetors.

ASIDE: if you are going to keep this 1987 Mercury 90-HP engine running, you will probably need to learn how to perform basic repairs yourself.

Give me advice on how to replace the 1987 Mercury 90-HP two-stroke-power-cycle engine's ignition system. The original ignition system is loose and is damaged.

The "ignition system" includes many components. Here is a list:

• three spark plugs
• three spark plug wires
• three spark coils
• the stator, where there is a coil winding that generates the spark coil primary voltage
• the stator, where there is a coil winding that generates the spark trigger voltage
• the switchbox that controls the spark generation; in some models there are two switchboxes, one for low speed and a second for highspeed engine operation

To replace all of these components on the basis that something is "loose" in not a good method of repair.

If something is "loose" the general repair procedure is to tighten whatever is loose.

Q3: how did you determine that there is "damage" to the entire "ignition system"?

Q4: have you tested each cylinder for spark?