Mariner 90hp 3 Cylinder 2004 - Bogging Causing Insanity

Hi everyone. I have a 90hp Mariner 3 cylinder, 2 stroke, 2004. It was garaged for several years and after I acquired it and its boat, I was delighted by its condition. After months detailing and repairing the boat and running over the basics on the engine I put the boat in the water.

The engine bogs but it’s erratic. Sometimes it runs like a dream, gets beyond 30 knots and all seems perfect, it can even pull a water skier or a tube. At other times the power goes up and down like a yo-yo with a mind of its own. Often it won’t even get up on plane, or just cuts out. It is very erratic.

Out of the water, without any load, everything works, performance is faultless. I have eliminated fuel, fuel lines, tank, fuel filters (by using an external, brand new tank with new lines and filters), compression is 125 psi per cylinder, every fluid in the engine has been changed so it is fresh, and I have had an engineer check it over. Still no joy and it is driving me to the point of insanity.

Having scoured the internet, I believe I have narrowed down the possible issues to:

• Ignition stator failure on high side.
  
  
• Idle mixture/low speed mixture badly adjusted.
  
  
• Dirty jets or carburetors.
  
  
• Fuel pump or gaskets in fuel pump not working.
  
  
• Coils/CDM have issues.
  
  
• Reed valves are problematic.
  
  
• Electrical temp sensor failure.
  

Can anyone help me? Did I get the list of potential issues right?

 

Sounds electrical to me. Best I can offer.

Dirty carb is not going to only run well no load..

 

I see you posted on the marineengine.com forum, a good place to get advice.
A few things to think about, not knowing the details of your specific engine.
1, are you positive that the seals on the quick connect fuel connector at the engine and at the tank are good ? It's still a 16 year old engine and the o-rings could degrade from ethanol in fuel or just simple wear and tear. I have found that it does not always leak the same way, especially if the tank position or orientation changes or the amount the engine is steered away from center.

2, if you prime the engine with the bulb, does the bulb go hard when the carb is full? If not their is a bit of a leak on the engine, or the engine quick coupling. Air will leak in more readily than fuel will leak out. I found on the last outboard that I bought the fuel lines on the engine were rotten causing air leaks when running. They leaked fuel when priming, but you couldn't see it unless you took the engine cover off.

3, sometimes the ignition coil will tend to die when its hot. Haven't had it on an outboard, but on a tractor. Would run great at idle or under load when cold but as it would heat up the secondary winding must have been arcing internally and it would run worse and worse till it quit. Let it cool down and it would start up again...

 

Thank you both. I am going to have the carbs cleaned anyway as a precaution, and to answer the point about fuel lines and quick connects, these have all been changed. What I am a little confused about is how you test coils/CDMs or do I just change them?

 

The timing light suggestion is a way to check if ignition is consistent on each cylinder. Sometimes it's possible that there could be a built in rev limiter that is malfunctioning. I seem to recall such was the case on a Briggs and Stratton air cooled outboard. Also investigate the dead man switch, although that should cut ignition off altogether. Sometimes the way something fails intermittently can be maddening

 

Thank you Keith, I am adding all these suggestions to the list, and I have a retired marine engineer helping me work through these from a practical perspective - it's my first engine...

 

Do you have the workshop manual, and have followed the trouble-shooting procedures ? It's a must-do. Otherwise it is too much like guesswork. That's what the late doyen of these forums, PAR (Paul Riccelli), used to tell people, and he was spot-on, you must have a good workshop manual at hand, you may need some basic electrical test gear like a multi-meter, to check the electrics for faults. Otherwise, it is akin to masochism, and replacing expensive parts without really knowing they are the cause, inadvisable.

 

You mentioned the engine had been unused for a long time. Rubber components, like fuel pump diaphragms often deteroriate when dried out for long periods. A leaking fp diaphragm may cause erratic behaviour.

 

You should check the stator before changing any parts. If the output is not correct, nothing will work correctly. You will need an analog tester. One of the indications of a cracked magnet on the flywheel, which can also be a problem, is that the voltage output has two peaks close together instead of a single one.
Page 39 - Mercury Mariner outboard ignition test specs https://www.outboardignition.com/page39.asp

 

Absolutely essential to be using a workshop manual, anyone who baulks at the cost, can't afford a boat.

 

Thank you Gonzo and Mr Efficiency. I have been looking for a workshop manual for ages, no-one seems to have manuals or even the chart which Gonzo so helpfully linked to for my engine - a 2004. I have found someone to perform the tests but I am now looking for the data I need to run the tests. Much harder than I expected but I am very grateful to everyone for pointing me in the right direction.