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Mariner 90hp 3 cylinder 2 stroke 2004 - Bogging Driving Me Crazy

MaltaBoating

New member
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?

 
Hook up a timing light to #1 cylinder.------Tape the light on a seat so you can observe the light.----Take boat for a spin.----Test all 3 leads.----That may help determine if it is a spark issue or something else.----Reed valves would not be intermittent.
 
Hook up a timing light to #1 cylinder.------Tape the light on a seat so you can observe the light.----Take boat for a spin.----Test all 3 leads.----That may help determine if it is a spark issue or something else.----Reed valves would not be intermittent.

Thank you, very kind. When you say test all 3 lead, do you mean all three cylinders? Sorry, never used a timing light before.
 
Thank you, very kind. When you say test all 3 lead, do you mean all three cylinders? Sorry, never used a timing light before.

Would work best with a helper. Go to most any auto parts store and get an "Ignition Timing Light" that operates on 12v, not household AC power. Read the instructions.

#1 is the top cylinder. The light illuminates when a pulse is sent to a spark plug to fire it. Don't look directly into the end; point it at a reflective surface....noon isn't the best time to see it properly...obviously. The rate of flashes varies with engine revolutions per minute (RPM). Flashing should be constant for a given rpm and should be the same for all cylinders. If not, like at some rpms or for certain cylinders you don't have constant blinks you have trigger/spark/spark-plug problems.

That being the case you didn't mention changing the spark plugs. That needs to happen. I have personal experiences with CDMs (the black modules with the high voltage lead that connects to the spark plug) having problems on that engine. After you install new plugs, if the problem still exists and you have intermittent firing on one cylinder, remove that CDM and swap it with another cylinder. Repeat your testing and if the problem is now on that different cylinder, the module that you moved is bad.
--------------------

Dirty carb surely can be a problem but needs some conditions...see below.

Low speed adjustments are the only ones you can make and the manual tells you how to properly adjust....your problem doesn't lend itself to that as a root cause. With them maladjusted, the problem is continuous, not hit and miss.

Fuel pump should pump or not pump. Checking it is easy, just squeeze the bulb when the problem exists....if recovery is immediate, then put a kit in the fuel pump. That doesn't support your problem either.

Racer covered the reed valves.....not them.

Temp sensor is a thermal switch embedded in the water jacket cover which surrounds the spark plugs. It is normally open and at about 195F or above it shorts, grounding the warning horn and a light if your boat has a dash light in addition to the horn. It just makes noise, it does nothing to the engine operation. Regular thermostat opening temp is 143F...a stuck stat wouldn't cause an intermittent problem. No help here.

In short, as in your list, CDM or stator are your best places to start your search....after you install new plugs. I'd opt for the CDM as the culprit over the stator due to your intermittently intermittent problem.

If your timing light tests fail to produce a smoking gun, pull the carbs and clean and install kits. Take a cotton swab and swab out the black fuel line that connects to each carb. If you find little black specks on the swab, that's fuel line degradation...obviously parts will be in your carb....I have found them jammed in the high speed jet located in/around the high speed jet in the bottom of the bowl and little pieces in the top of the carb. Replace your engine fuel lines, at least between the fuel filter and the carbs.
 
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Thank you Texasmark, very very much appreciated along with the suggestion from Racerone. I am working through this all to make sure I understand it fully.

Would work best with a helper. Go to most any auto parts store and get an "Ignition Timing Light" that operates on 12v, not household AC power. Read the instructions.

#1 is the top cylinder. The light illuminates when a pulse is sent to a spark plug to fire it. Don't look directly into the end; point it at a reflective surface....noon isn't the best time to see it properly...obviously. The rate of flashes varies with engine revolutions per minute (RPM). Flashing should be constant for a given rpm and should be the same for all cylinders. If not, like at some rpms or for certain cylinders you don't have constant blinks you have trigger/spark/spark-plug problems.

That being the case you didn't mention changing the spark plugs. That needs to happen. I have personal experiences with CDMs (the black modules with the high voltage lead that connects to the spark plug) having problems on that engine. After you install new plugs, if the problem still exists and you have intermittent firing on one cylinder, remove that CDM and swap it with another cylinder. Repeat your testing and if the problem is now on that different cylinder, the module that you moved is bad.
--------------------

Dirty carb surely can be a problem but needs some conditions...see below.

Low speed adjustments are the only ones you can make and the manual tells you how to properly adjust....your problem doesn't lend itself to that as a root cause. With them maladjusted, the problem is continuous, not hit and miss.

Fuel pump should pump or not pump. Checking it is easy, just squeeze the bulb when the problem exists....if recovery is immediate, then put a kit in the fuel pump. That doesn't support your problem either.

Racer covered the reed valves.....not them.

Temp sensor is a thermal switch embedded in the water jacket cover which surrounds the spark plugs. It is normally open and at about 195F or above it shorts, grounding the warning horn and a light if your boat has a dash light in addition to the horn. It just makes noise, it does nothing to the engine operation. Regular thermostat opening temp is 143F...a stuck stat wouldn't cause an intermittent problem. No help here.

In short, as in your list, CDM or stator are your best places to start your search....after you install new plugs. I'd opt for the CDM as the culprit over the stator due to your intermittently intermittent problem.

If your timing light tests fail to produce a smoking gun, pull the carbs and clean and install kits. Take a cotton swab and swab out the black fuel line that connects to each carb. If you find little black specks on the swab, that's fuel line degradation...obviously parts will be in your carb....I have found them jammed in the high speed jet located in/around the high speed jet in the bottom of the bowl and little pieces in the top of the carb. Replace your engine fuel lines, at least between the fuel filter and the carbs.
 
Thank you Texasmark, very very much appreciated along with the suggestion from Racerone. I am working through this all to make sure I understand it fully.

So, I got the carbs/jets cleaned, and installed brand new plugs and checked the spark. I found someone to help me in a practical sense. I still have questions (apologies) but I'm excited to be making progress:

1) Texasmark said "After you install new plugs, if the problem still exists and you have intermittent firing on one cylinder." - how would I know I was intermittently firing on one cylinder? Does this mean using the timing light again?

2) Where do I get the numbers to test the stator against? I've been looking for a workshop manual or anything for a 2004 engine but never found anything with the right date for my serial numbers. Also do I need more than a multimeter to test a stator?
 
If you can't find what you want by shopping here...top of the page, if a CD will work for you try this: https://www.ebay.com/itm/Mercury-65...e-Manual-CD-/254573931625?hash=item3b45c9ec69

Otherwise ebay has hard copy manuals. My hard copy was $60 and was the 2015 revision of this, the last manual they made for this series of engines...to my knowledge. The answer to your questions are therein and yes you need a multimeter. I have used the Harbor Freight cheap ones for years and have no regrets. Sell for about $6 bucks, but other retailers sell them...HD and Lowes probably.

And yes on getting back with the timing light. Working at night makes detection much easier.
 
Dear Texasmark and Racerone - we took all the electrics and the carbs apart, and you were right. The stator was fried and so was the voltage regulator and we believe there were also issues with the carb needles. So I am looking for a replacement stator, and have found needles, gaskets and a voltage regulator. I have one more question, forgive me - these stators are hard to find in the UK, plenty in the USA but they won't get to Malta in time for the season. What I have found is stators with near identical part numbers, many sites/suppliers say they are interchangeable, it is said they are suitable for engines from 30hp to 90hp+

For example, instead of part number 832075A 4 they are part number 832074A 5 - I am being idiotic but trying to find alternatives?

Question aside, a huge thank you, I'd never be close to a solution without you both.
 
In browsing engines, a range of engines are listed and parts therein shown. I looked up my 2002 by serial number and both 832074 and 832075 are shown for that series of engines. I then went to the end of that range of engines and looked up part numbers and the differences were only in later numbers having dash numbers. Without them changing the part number and only the dash number, one would expect cosmetic differences, or added things like zip ties, cable jumper wires added and such. Both numbers wouldn't be listed if they didn't work. On the cosmetic differences you can find a zip tie or make your own jumper wires if too short to fit.

"Carburetor kits" are identical for both series of engines yours and mine: 8236354. Doesn't matter the specific orfice size of the carb as far as the kit is concerned.......ensure that you keep the carbs straight.....The carb you took off the top is #1 and goes back in the top slot....etc.....jet sizes are different for the different positions on the engine.

On cleaning the carbs, check for little black specks in the bowl area and on the top under the removable plate. If you find any you need to replace your black rubber fuel lines. In cleaning and soaking the carbs, run a fine. soft wire....strand of copper electrical wire works great.....through any hole you can find. Remove the high speed jet...copper plug in the bottom of the bowl...check for debris and blow everything out good with compressed air....reinstall.

Be sure to set the float properly...should have a guage in the kit...basically, if you install the bowl, clip, and needle valve, with the carb in the normal position, the float will be hanging down. Flip the carb over, bottom side up, and look at the casting surface where the bowl mounts and look at the centerline of the float.....the proper setting will usually get the center line of the float parallel to the flange on the carb that mates to the bowl.

You said you changed fuel filters....did you include the one in the engine compartment? If you didn't, do it. If you did/when you do and still have it cut it open. If full of brown chips (fuel by products in supply line to engine) and/or black specks (engine fuel lines) replace fuel lines back to the fuel tank along with a new squeeze bulb.

On setting the low speed adjustment, start with 1.5 turns out from 2 finger snug. Get on the water and attempt to get on plane....firewall the throttle. If it bogs, slightly open each....screwdriver width or so. Continue till it takes the throttle and gets up. If you get to 2 turns and still bogs, you have other problems.

This may help: https://public-mercurymarine.sysonl...=GuestNA_KEY=NA_KEY_VALUElangIF=englangDB=eng

It's the complete parts catalog so it will take time to load. Get your serial number off the right side of the transom clamp bracket..easily seen standing behind the starboard side of the engine. I just guessed at yours.
 
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