Mercury 90-HP Two-stroke-power-cycle Engine: Odd Starting

[dvanaken]

[A c.2003 Mercury 90-HP two-stoke-power-cycle engine exhibits unusual starting problems. The problems occurs in the following situation:

• the engine is started and run for about three minutes propelling the boat;
• the engine is shut off for about one hour;
• the engine is restartsed without using any "choke" (fuel enrichment);
• the engine idles for about four seconds and stalls;
• the engine cannot be restarted.

In trying to restart the engine, "choke" (fuel enrichment) was used. Gasoline was seen floating on the water. The engine never even sputtered.

If the engine sits overnight, the next morning the engine will start immediately. This behavior has occurred at least three times. Prior to 2023, the problem did not occur.

At the last occurrence of no-starting, I checked for presence of spark on all cylinders using a separate spark plug as an indicator; there was spark on all cylinders.

Based on that test, my inference is the cause of the starting difficulty is due to a problem in the carburetors. But with three independent carburetors, I would expect at least a sputter in one cylinder.

Q1: what is the cause of a failure to fire on all three cylinders?

Since there is gasoline on the water, I do not suspect the cause is related to the fuel pump.

I think the cause may be due to a stuck float valve in a carburetor.

Q2: if the cause of the starting problem is due to a stuck float valve in one carburetor, why is there no firing on the other cylinders?

The last time the no-start situation occurred, the engine was allowed to sit a further 10-minutes, and then it started without use of "choke" (fuel enrichment), but the engine seemed to be running on only two-cylinders for about 30-seconds, then the engine appeared to "clear out" and "open up" [that is, appeared to be running on three-cylinders].

Once the engine is started and running, the performance is normal.

I'm not near any qualified Mercury dealers so I'd like to diagnose the cause of the hard starting myself.

Give me new ideas about what to look for as the cause of the hard starting.

Q3: has another c.2003 Mercury 90-HP engine ever exhibited this behavior?

[jimh]

If the carburetors in the c.2003 engine have never been disassembled and cleaned, after 21-years it may be time for a look at them.

Q4: when the engine finally starts, is there an unusual amount of smoke in the exhaust?

[dvanaken]

Thanks for replying.

I'm not aware of unusual smoke, but I haven't directly paid attention. This motor often smokes when started. Possible oil injection is too high?
I remember having to lower the injection rate after some number of running hours. I could try to dial it back a bit. Is it possible to set it too low?

Regarding cleaning the carbs, I am basically planning to do that unless another clue surfaces, but there are less than 200 hours on the motor so I keep thinking the carbs can't be too bad. I ordered gasket sets and will clean and reassemble all 3 carbs.

Anything that this particular set of symptoms could be? Start fine->start for a few secs and stall->fail to start.

Thanks

[jimh]

Regarding the presence of more than normal smoke on engine start: my thinking was along this path:

• I assume your engine has carburetors, based on your narrative;
• the fuel that is delivered to the carburetor bowl is fuel that has already been mixed with lubricating oil;
• when the engine sits unused the gasoline part of the gasoline-oil fuel mixture tends to evaporate;
• due to loss of gasoline to evaporation, the mix ratio of the fuel in the carburetor bowls become more rich in oil than normal;
• when the engine starts and is combusting the fuel with a rich mixture of oil in the gasoline, the effect is generally to observe more smoke in the engine exhaust than normally seen when the fuel being burned is at the normal mixing ratio.

Some other possible reasons for poor starting

Also, if the engine is stored with the engine NOT in the normal running position, but instead is tilted upward so the engine is nearly horizontal, the effect is often to have gasoline spill out of the carburetors due to flow of fuel by gravity. If the carburetor bowls are empty of fuel, starting the engine may be difficult. A good practice to follow when trying to start a carburetor engine that has been moved to a nearly horizontal position is to first squeeze the in-line fuel primer bulb in the fuel supply hose until you feel that the fuel line is exhibiting some resistance, indicating the downstream hoses are now filled with fuel. Tilting up the engine and not priming the fuel hose primer bulb could be factors in the poor starting behavior.

Another concern with fuel is the ethanol content. Using diluted gasoline fuels in which ethanol has been added in a 1:9 ratio with gasoline can cause harm to fuel system components like seals, gaskets, non-metal valves, and anything made with rubber. If the fuel line between the fuel tank and the engine has any part of its hose that has a silver-metallic outer jacket, you should discard that hose. Hoses with a silvery metallic finish are known to have poor resistance to gasoline with ethanol content, and are known to cause problems with outboard engines. The essential problem is the inner line of the fuel hose separates from the rubber part of the hose, and becomes narrow and restricts fuel flow to the engine. Replace any silvery-metallic hose between the fuel tank and the engine inlet fitting with a proper rubber hose with excellent resistance to ethanol fuels.

Generally the owner's manual for a particular outboard engine using gasoline will give explicit instructions on the steps to take before attempting to start the engine, particularly in regard to the position of the throttle, use of the primer bulb, when use of fuel enrichment ("choke") should be employed, and so on. Following the exact steps given by the manufacturer generally is the BEST way to start the engine, and deviating from the recommended starting procedure often produces less efficient starting.

[dvanaken]

Thanks for your ideas

[MarkCz]

I have a 1987 Mercury 70-HP two-stroke-power-cycle engine and had a similar situation about seven years ago with difficult starting after sitting for 30 to 60 minutes; I also would observe oil on the surface of the water. I replaced the fuel pump and the engine ran perfectly with easy starting and no more oil visible on the water.

[dvanaken]

Mark - Many thanks. I was hoping find someone who had these same symptoms. I ordered a replacement pump and it arrives in a few days. Hope this does a new fuel pump is the remedy to the erratic starting.

[jimh]

The fuel pump on older classic two-stroke-power-cycle engines is usually a diaphragm pump that is operated by an air motor. The air motor is powered by pressure variations in the engine crankcase.

The diaphragm in the fuel pump is typically rubber. Although there are no comments about the type of fuel used, if any gasoline fuel with dilution with ethanol has been used, there is a further reason to suspect that the rubber components in the fuel pump have been negatively affected. However, even without ethanol fuel, 21-years of exposure to gasoline could easily result in some damage to the rubber in the fuel pump. Replacement of the fuel pump at age 21 would not be out of the ordinary service life of such a fuel pump.

[dvanaken]

Mark and Jim-

Thank you for the tip on the fuel pump. After messing with a poor quality aftermarket pump, I bought the Mercury rebuild kit and in 10 minutes had a rebuilt pump. The diaphragm membranes were stiff, but seemingly functional. The check valves were ok. I did find 10-12 little hard pieces of what may have been varnish from the lines. It's possible one or more could be catching in the check valve and limiting the pumping operation intermittently.

I decided to replace all the lines starting at the tank. All is well, no leaks, and so far have had 8-10 prompt starts. So I'm cautiously optimistic that this was solved with $60 in parts.

Thanks again for the advice.

Dale

[MarkCz]

Thanks for letting us all know what you did to fix the problem of your Mercury 90-HP engine not being able to reliably be started.

I am pleased to hear your engine starting is much easier now.

[dvanaken]

I just returned from using the boat in what had been the typical failure situation: I motored across the lake to the restaurant, had wine and snacks, then I hoped the Mercury 90-HP engine would start.

The engine did start, just fine. My passengers were happy.

Also, I think the engine is now running smoother than before. Perhaps the fuel pump may have been somewhat under-supplying fuel to the engine.

[Don SSDD]

I’m late reading this but when I see a 21 year old outboard with 200 hours, ie 10 hours per year, I would suggest you should run some Sea Foam or similar at high levels in your fuel, and run it at a high throttle setting to clean out your carburetors and the non rubber bits in your fuel system. You may have improved things with the fuel pump kit and lines replaced but whether 2 stroke or 4 stroke, 10 hours a year can lead to fuel system problems.