Help us before my husband throws our Mercury outboard in the water!

[alaskaflyfish]

Just so you know, not all mechanics are created equal. A few more questions need to be asked. Does your engine begin to have this issue after it is up to operational temp? If so, it could be a heat related ignition issue. This is very difficult to diagnose. Defective coils will break down and increase resistance when heated but will function properly when cool. Will it run and idle properly immediately after the high speed condition manifests, or do you have to wait for a cool down period before the motor will run again?

[Vasco]

Clean the carb taking extra care with the jets. Quite easily done. Don't rely on "mechanics".

[Heezea]

I had a similar problem on my boat motors. I have two suggestions.

1. Make sure your carb jets are adjusted correctly.
I'm not sure if that specific engine has jets, but usually a carb has very small parts that are bronze needle-like "jets". These spray the fuel into the carb. Usually these can be adjusted by screwing out or in to allow more or less fuel in. People usually keep track of how open or how closed the jets are by counting the rotations of the screw until it becomes firmly screwed in (use the lightest of force, the bronze is soft). So firmly screwed in is shut. A standard "openness" is two full rotations (360° + 360°) open starting from the fully shut position.

There may be two jets, one for the idle and one for the open throttle. Since you're having issues with open throttle, that jet may need to be adjusted. If you're not sure which one to adjust, just pick one. First, turn it fully shut and count the rotations; record the rotations to remember how open it was initially set. Then open it back to where it was + one half of an additional rotation (180°). If your idle sounds louder or more vigorous but your open throttle doesn't improve, you may have adjusted the wrong jet. Reset the one you adjusted back to what it was and adjust the other one to the +180° position.

2. Make sure you throttle cables are adjusted correctly. This is a very similar issue in that the handle (throttle) controls the amount of air and fuel, similar to the carburetor. When you rotate the handle, two things happen. More fuel is added and more air is added. If too much air is added and not enough fuel is added, the engine could stall. Make sure that as your rotate the throttle, the little lever that moves back and forth travels the full distance that it can travel (check the full distance by pushing the lever by hand).

[P3sailor]

I'm guessing this is a two-stroke engine, if so, I would do as er9 suggested and check the carburetor jets. The oil from the gas mix tends to build up in carburetor jets and can cause any number of issues. Carburetors are relatively easy to rebuild, and I've found that even though the jets may appear to be clear of any obstructions they need to cleaned using a "mechanical" device such as copper wire or anything softer than the brass jet. Jets are cheap so you may want to purchase a few just to have some spares. You also may opt to purchase a carburetor rebuild kit for your engine, it will have all the o-rings, seals, needle valve, and more for you to do your own repair. If you're careful you'll be able to use many of the items over several times.

[Colin A]

Well if you make sure everything on the fuel side is good next is spark. In my experience the coil is the most likely given the symptoms.

You can sometimes find the factory service manuals on ebay. Not sure if there still is but years ago there was a guy selling downloads of the factory manuals cheap. Might have been shutdown by now thou.

[alaskaflyfish]

Originally Posted by Vasco

Clean the carb taking extra care with the jets. Quite easily done. Don't rely on "mechanics".

That's very bad advice Vasco. I've had more business from the "do it yourself" crowd repairing their "mess ups" than I have from mistakes of other trained mechanics and technicians. Independence is a good trait but the OP may have their own limitations. The object here may be the OP finding quality help that will do a post-maintenance follow -up to correct the still existing condition.

[Cadence]

Originally Posted by Jammer

Usually, problems with the motor failing abruptly after it warms up are ignition related. I am diagnosing one this weekend on my 1956 Johnson 3hp. Newer motors, with electronic ignition, are susceptible to the same.

Re: 3Hp 1956 OMC. A pinhole in a coil, which is under the flywheel and arcs to the flywheel, can be a bitch to find. FWIIW.

[Empty Pockets]

As others have already suggested, if it starts and runs slow ok, but falls on its face under load, I’m looking at coils...

[Jeanneau 45.2]

Very cosmic. I have a 2002 6HP Mercury 2 stroke which developed in the past week what appears to be a very similar problem. It starts on 1 pull and idles perfectly, but shortly after getting under way at speed, it also slows down to the point of "gasping for breath" (I know, I am putting a human symptom upon an inanimate object). I have drained all fuel from the carb, drained the 3 gallon fuel tank, and checked that I have fuel pressure at the hose / engine connection. I have not yet found the root cause, BUT I HAVE FOUND A GOOD WORKAROUND. PARTIALLY PULLING OUT THE CHOKE WHILE INCREASING THROTTLE TO CRUISING SPEED ALLOWS NORMAL PERFORMANCE. I have gone several miles multiple times without problems and my fuel consumption is in the normal range. This rules out, IMHO, spark and air issues, and must relate to a jet or other carb issue. One I get to a port where I can work on the outboard out of the water, I'll tinker. Suggestions welcome.

As an aside, how do you ensure that your dingy and outboard are never stolen in the Caribbean ... buy a Mercury Actually mine has been problem free until this recent malady.

[Vasco]

Originally Posted by alaskaflyfish

That's very bad advice Vasco. I've had more business from the "do it yourself" crowd repairing their "mess ups" than I have from mistakes of other trained mechanics and technicians. Independence is a good trait but the OP may have their own limitations. The object here may be the OP finding quality help that will do a post-maintenance follow -up to correct the still existing condition.

The OP already spent $300 on a "mechanic" and the problem was not fixed. You want him to spend more on more "mechanics"? Cleaning a carb is not beyond those who can read or watch a video. Just takes a bit of patience.

[alaskaflyfish]

Originally Posted by Jeanneau 45.2

Very cosmic. I have a 2002 6HP Mercury 2 stroke which developed in the past week what appears to be a very similar problem. It starts on 1 pull and idles perfectly, but shortly after getting under way at speed, it also slows down to the point of "gasping for breath" (I know, I am putting a human symptom upon an inanimate object). I have drained all fuel from the carb, drained the 3 gallon fuel tank, and checked that I have fuel pressure at the hose / engine connection. I have not yet found the root cause, BUT I HAVE FOUND A GOOD WORKAROUND. PARTIALLY PULLING OUT THE CHOKE WHILE INCREASING THROTTLE TO CRUISING SPEED ALLOWS NORMAL PERFORMANCE. I have gone several miles multiple times without problems and my fuel consumption is in the normal range. This rules out, IMHO, spark and air issues, and must relate to a jet or other carb issue. One I get to a port where I can work on the outboard out of the water, I'll tinker. Suggestions welcome.

As an aside, how do you ensure that your dingy and outboard are never stolen in the Caribbean ... buy a Mercury Actually mine has been problem free until this recent malady.

By your description it sounds like a fuel issue due to a change in condition based on a fuel/air ratio adjustment. Pulling the choke reduces the carb throat air intake enriching the fuel mixture.

[travellerw]

Originally Posted by Jeanneau 45.2

Very cosmic. I have a 2002 6HP Mercury 2 stroke which developed in the past week what appears to be a very similar problem. It starts on 1 pull and idles perfectly, but shortly after getting under way at speed, it also slows down to the point of "gasping for breath" (I know, I am putting a human symptom upon an inanimate object). I have drained all fuel from the carb, drained the 3 gallon fuel tank, and checked that I have fuel pressure at the hose / engine connection. I have not yet found the root cause, BUT I HAVE FOUND A GOOD WORKAROUND. PARTIALLY PULLING OUT THE CHOKE WHILE INCREASING THROTTLE TO CRUISING SPEED ALLOWS NORMAL PERFORMANCE. I have gone several miles multiple times without problems and my fuel consumption is in the normal range. This rules out, IMHO, spark and air issues, and must relate to a jet or other carb issue. One I get to a port where I can work on the outboard out of the water, I'll tinker. Suggestions welcome.

As an aside, how do you ensure that your dingy and outboard are never stolen in the Caribbean ... buy a Mercury Actually mine has been problem free until this recent malady.

Thats an easy one. If the problem goes away with the choke partially pulled out, then its %100 a plugged jet.

Dismantle the carburetor and clean all the jets and carb body thoroughly. This includes soaking the jets in carb clean for 30 min or so and then blowing out with compressed air. After that, hold the jets up to the sun and make sure you can see light through all the holes. In rare cases a jet cleaner (basically a tiny wire) needs to be passed through the holes as the particle is not soluable in carb cleaner (a tiny piece of sand). However, EXTREME CARE is required as the wire can break off essentially destroying the jet.

P.S. No, spraying carb cleaning down the throat of the carb will not solve that problem. It must be dismanlted.

[Vasco]

Originally Posted by travellerw

In rare cases a jet cleaner (basically a tiny wire) needs to be passed through the holes as the particle is not soluable in carb cleaner (a tiny piece of sand). However, EXTREME CARE is required as the wire can break off essentially destroying the jet.

.

I use a 50# test monofilament line for cleaning the jets. less likely to scuff or damage.

[alaskaflyfish]

Originally Posted by Vasco

The OP already spent $300 on a "mechanic" and the problem was not fixed. You want him to spend more on more "mechanics"? Cleaning a carb is not beyond those who can read or watch a video. Just takes a bit of patience.

Like previously stated "not all mechanics are created equal". You assume cleaning the carb jet is within the OP's scope of limitations. (maybe not)? I wonder how many non-professional mechanics here would have pulled the carb and serviced it first without first checking to see if it was a ignition issue (be honest). Spend the time and money on the carb only to find out it was not the issue? Fortunately, it is almost always the carb and most owners just ignore the ignition, hoping and praying it will never have a malfunction. The mechanic attacked the usual suspect based on an owners description of events when the motor is brought to him/her. Ignition issues can be very difficult to determine (especially when intermittent) even for a professional.

[Mojadito]

Let him throw it in, then buy yourself a Yamaha, you’ll be much happier in the long run