Mercury 90-HP Starting Problems

[dvanaken]

My 2002 Mercury 90 two-stroke-power-cycle outboard engine with less than 200 hours has failed to start twice in the last week. To me this indicates the cause of the no start is due to the gasoline fuel, or due to the carburetors, or due to the spark ignition.

Q1: where can I start the diagnosis of the cause of the no start condition?.

BACKSTORY

The Mercury 90 typically will initially start, but stall after running for five seconds. Any attempts after that to start will fail without even even a hint of ignition. Fuel drips into the lake.

The first occurrence of non-starting start like this, I waited five minutes before trying again. I set the throttle at half-throttle. The engine begrudgingly started, but ran very roughly. I got underway; I could feel that we only had two cylinders firing, then the third cylinder came on. My impression was a carburetor was flooded and was clearing out, or a spark plus was bad.

I replaced all three spark plugs.

The second occurrence of rough starting was similar. Gasoline dripped into the lake. On this occassion, I waited 30-minutes before trying again, but the engine did not start. A buddy towed me back to my dock.

[Back at the dock] I pulled off the air box cover and tried tilting the motor up. A steady but small stream of gasoline ran out of the lower carburetor air intake. The fuel continued to spill for 15 seconds, and then I tilted back to level.

I let the engine sit overnight. The next morning it started immediately, although it's idling rough and I didn't leave the dock. I should point out that in between these two incidents the engine started and ran normally.

[Survey question about how often this has happened to any reader has been removed.]

It seems like the abundance of fuel indicates a fuel control problem.

Q2: what regulates fuel flow to the carburetors?

Q3: is excess fuel a normal result of the repeated attempted starts and this is a spark problem, especially since when failing to start there is not a hint of anything firing?

If the cause is electrical, it's obviously intermittent, and has not ever happened when running - only when starting - so it's hard to imagine a root cause.

Q4: where do I look?

Q5: fuel or spark?

Q6: what can I check without waiting for another failure?

I should add that the boat lives here permanently; there is no trailer; there is no trustworthy Mercury service on the lake. I'm probably on my own here.

Many thanks for ideas.

[MarkCz]

I have a 1987 70-HP three-cylinder two-stroke-power-cycle engine that exhibited similar problems a few years ago. I also noted oil in the water around the engine.

I replaced the engine's fuel pump and [my boat's fuel supply line] check valve: that fixed the problem, without great expense.

A fuel pump can be rebuilt at less cost than a new fuel pump. I didn't want to deal with all the small parts.

[My engine did] not have [loss of spark ignition].

If your engine has no spark, [the fuel supplied to the carburetors] will be pushed out the cylinder exhaust port as unburned fuel-oil mixture, down to the propeller exhaust hub, and into the lake water, creating an an oil sheen.

[jimh]

Q1: where can I start the diagnosis of the cause of the no start condition?
Q2: what regulates fuel flow to the carburetors?
Q3: is excess fuel a normal result of the repeated attempted starts and this is a spark problem, especially since when failing to start there is not a hint of anything firing?
Q4: where do I look?
Q5: fuel or spark?
Q6: what can I check without waiting for another failure?

All your questions should be answered in the following advice.

Fuel flow into the carburetor bowls is controlled by a float mechanism. The fuel pump runs from from crankcase vacuum pulses, so it tries to to pump fuel all the time the engine is running. Before starting a two-stroke-power-cycle engine, the operator must prime the fuel system using the in-line primer bulb to get the carburetor bowls full of fuel. When the fuel level in the carburetor bowl is high, a float valve mechanism closes a valve that stops more fuel from entering the bowl. A common problem is the float mechanism becomes stuck. The carburetor float valve can stick in the open position, causing the carburetor bowl to be over-filled with fuel; or the float valve can stick in the closed position, causing the carburetor bowl to run out of fuel. Either condition can affect the engine operation.

Fuel from the bowl is sucked into the air stream by the Venturi effect. The rate of fuel mixing with air is determined by an orifice that restricts the flow.

When you tilt up the engine to be nearly horizontal, in most instances any fuel in the carburetor bowls will spill out via the carburetor throats, and end up spilling into the engine cowling pan, eventually dripping overboard into the sea.

If unburnt fuel is flowing out of the engine while the engine is in a vertical or operating position, fuel should not be spilling out of the carburetor bowls--unless the float valve is stuck open. Or, there could be a leak somewhere downstream of the fuel lift pump in the fuel distribution system. You could look for a leak by using the primer bulb to pressurize the fuel delivery system, then carefully watching to see if fuel leaks out somewhere downstream of the primer bulb.

In order for the engine to start there must be three elements present:

• air
• fuel
• spark

SPARK
A very good, very simple, very inexpensive way to test for spark is to use a LISLE IN LINE SPARK TEST tool. Insert the tool in-line with the high-voltage lead to a spark plug. If there is spark you will see the arc jumping the gap in the test tool. Try this on each cylinder individually. Do not just unplug the high-voltage wire from a spark plug and run the engine with the high voltage wire unterminated, as it could cause harm to the spark generating components.

The spark voltage being generated can be very high, and in some instances, where there is deterioration of the wiring and wire insulation or in the spark generating coils, the spark voltage can be bleeding off to ground BEFORE it gets to the spark plug. A very careful visual inspection of all the wiring, insulators, coils, and other elements of the spark ignition system can detect burning or discoloration caused by the short-circuiting of the voltage and arcing.

FUEL AND AIR

The fuel and air must be present in the proper ratio. If either there is too much fuel compared to air or not enough fuel compared to air, cylinder ignition will not occur.

AIR

The air system is rather simple, so a problem with air flow into the cylinder is unusual.

FUEL

More likely the problem with the fuel-air mixing ratio is either too much fuel or not enough fuel. Fuel dripping out the engine after a failed start attempt is probably an indicator that the fuel-air mixing ratio had too much fuel.

A further indicator that the cause of the no-start was too much fuel in the fuel-air mix is your report that letting the engine rest for some period of time from 10 to 30-minutes usually restores the ability to start the engine. This is likely due to fuel evaporating into the air or draining away from the carburetors during the resting time.

A first step in locating the problem will be to remove the engine cowling and maintain a close visual monitoring of the engine, particularly in the area of the carburetors and fuel supply hoses, while you attempt to start the engine and immediately after the engine ignites and runs momentarily before stalling. The odor of gasoline in the air is easily detected, so your nose may help you locate the source of any gasoline fuel that is leaking.

Contaminated fuel with water present in the fuel can also affect engine starting and running. To eliminate the fuel and the fuel tank, if possible operate the engine from a different fuel tank with fresh fuel, such as a 6-gallon portable fuel tank.

[Jefecinco]

Q7: how old is the fuel in the tank?

Old fuel that has not been stabilized correctly should be replaced. Smelling the fuel can provide an indication of age.