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Old 03 July 2022, 14:15   #1
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Mercury 15 hp 2 Stroke cutting out

Hi all, following on from the carb challenges this engine is becoming more frustrating

Carb is re built. Starts first pull in the tub. Put it on the water and under load and it runs fine one day dies and wonít start the next.

Back in the tub now. I figured it may well electrical so to rule that our states up on tick over. Pulled the spark plug caps off.

It will run and tick over on the top cylinder with the bottom cap removed. Cuts out if you reverse that.

So Iíve swapped the coils over. Still runs top cylinder not on the bottom

Swapped plugs over same result

Checked both cylinders are firing. They are snd it wonít rev well in neutral unless both cylinders are connected.

It will start on the top cylinder only

Very confused nowÖÖ. I assume itís dying under load as the top cylinder is doing all the work and flooding the bottom cylinder leading to a stall (Iím guessing).

Iím at a loss now as itís not the coils, plugs are good, when warm runs great. Plugs are a light tan so mix and jets are right.

Summary is starting is sometime difficult. Engaging gear sometimes kills the engine making starting hard, symptoms are like itís bogging down, any advice?
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Old 03 July 2022, 14:52   #2
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Low compression on the bottom cylinder perhaps
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Old 03 July 2022, 15:32   #3
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Just ordered a compression tester as I donít have one. When it runs it runs well though with no excessive smoke. Maybe though
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Old 04 July 2022, 06:12   #4
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Always worth doing a compression test.

The intermittent nature is the strange bit.

There's always a chance that the inability to run just on the bottom cylinder is separate to this and you've got low comp but the test will rule that out or in as a possible scenario.

It's also hard to imagine compression coming and going.

If the electrics are tested and spark good then it sends us back to the air/fuel thoughts and what could vary between letting too much air in or not enough fuel in periodically?

Have you been able to notice if the lean running is correlating with anything else like high temps etc?
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Old 12 July 2022, 15:17   #5
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Compression test is good both cylinders are the same. Took the carb off again and cleaned. Bulb pumps hard no leaks anywhere.

Put it back on the water started first pull ran it out into the sound all good up snd down revs pulled well and responded correctly

Back into the harbour slow running back fired then died and would not start.

Then it did start after a while now will tick over but dies as soon as throttle is applied

Iím convincing myself now itís the switch box giving poor spark to the bottom cylinder. No way to test it other than to buy another and see if that cures it.

Iím at a total loss with this one
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Old 12 July 2022, 17:28   #6
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Fuel pump ok?
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Old 12 July 2022, 18:22   #7
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Think so rebuilt it, it will tick over no problem which must mean itís pulling some fuel. Doesnít want to rev in neutral easily. Changing the mixture isnít having much effect but Iím thinking thatís because the top cylinder is doing most of the work at tick over

Both cylinders are definitively sparking but the Engine will cut if I pull the top plug but runs if I pull the bottom one

Changed the spark plugs for new ones.
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Old 12 July 2022, 19:00   #8
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Quote:
Originally Posted by neil.mccrirrick View Post
Think so rebuilt it, it will tick over no problem which must mean itís pulling some fuel. Doesnít want to rev in neutral easily. Changing the mixture isnít having much effect but Iím thinking thatís because the top cylinder is doing most of the work at tick over

Both cylinders are definitively sparking but the Engine will cut if I pull the top plug but runs if I pull the bottom one

Changed the spark plugs for new ones.
Had similar with a Suzuki 25 fuel pump had a torn diapham
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Old 12 July 2022, 19:27   #9
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Itís a new diaphragm I do have another. If nothing else Iíll take the carb off again and swap the diaphragm
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Old 12 July 2022, 19:41   #10
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High tension lead or spark plug cap breaking down?
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Old 12 July 2022, 19:51   #11
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I swapped the coils and the leads, top cylinder still fires much better than the lower one
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Old 12 July 2022, 20:29   #12
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Ultrasonic clean the carbs.
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Old 12 July 2022, 20:41   #13
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Chapeau to you Neil, you’ve the patience of a saint. I haven’t but OB’s can drive you nuts. Not sure if anything in this below might help?

Reaching the end of my tether with outboards...non-runner…
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Old 12 July 2022, 22:21   #14
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Hi Neil, I don’t know if you remember but I had a similar issue with a Honda 15hp, it turned out to be a CDI unit.

I’ve got an ultrasonic cleaner here that I do carbs with. If you can be bothered with the run up the road you are welcome to drop the engine off and I’ll run the carb through the cleaner. I’ve got a few spare engines at the moment, I could lend you one to keep you going.
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Old 13 July 2022, 07:36   #15
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Thanks Alan very much appreciated. Iíve just ordered a cdi that I can return if it doesnít fix this

Thanks for the offer of the cleaner. I think someone on the island has one Iíll see if I can borrow that.

Very kind re outboard. Iíve a 6hp aux for the rib Iíll pop that on for now
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Old 13 July 2022, 07:38   #16
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Thanks Max Iíll re the thread. I am fed up with it running well in the bin then failing as soon as itís under load. Iím sure Iíll get there eventually
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Old 13 July 2022, 07:58   #17
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Read the thread thanks max. Plunger is good on mine. Iíve stripped and rebuilt this carb 4 or 5 times now. But only used carb cleaner. Off for an untraditional bath and Iíll swap the cdi out when the part comes.

Iíll update again after that
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Old 13 July 2022, 07:59   #18
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Untraditional??? Ultrasonic lol
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Old 13 July 2022, 08:13   #19
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Best of luck - f * * * i n g OB's!!! Argggggggggggghhhhhhhhhhhhhhhh.
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Old 14 July 2022, 15:34   #20
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Bit of an update. Cdi has not arrived yet and Iíve not took the carb off yet. Boats in the harbour so I thought Iíd try something.

Cold start first pull ticked over. Set off was stalling pulled the choke to prime and took off.

Engine running both cylinders no hesitation out into the sound back into the harbour. Slow running off the plane but bow still up all good

Bow down displacement speed engine cuts. Will start but wonít run and then wonít idle and dies

I still think Iíve poor spark at low revs on one cylinder? But equally maybe something else at play as it comes off idle before the high speed jet kicks in?

Iíll strip the carb again when I can and look at the primer specifically. Itís got free movement and is returning so looks on from the outside

Summary still perplexed
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