Mercury 15 hp 2 Stroke cutting out

neil.mccrirrick · 03 July 2022, 15:15

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 won’t 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 I’ve 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 won’t rev well in neutral unless both cylinders are connected.

It will start on the top cylinder only

Very confused now……. I assume it’s dying under load as the top cylinder is doing all the work and flooding the bottom cylinder leading to a stall (I’m guessing).

I’m at a loss now as it’s 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 it’s bogging down, any advice?

Lemonbonbon · 03 July 2022, 15:52

Low compression on the bottom cylinder perhaps

neil.mccrirrick · 03 July 2022, 16:32

Just ordered a compression tester as I don’t have one. When it runs it runs well though with no excessive smoke. Maybe though

TmMorris · 04 July 2022, 07:12

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?

neil.mccrirrick · 12 July 2022, 16:17

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 [emoji35]

I’m convincing myself now it’s 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.

I’m at a total loss with this one

jeffstevens763@g · 12 July 2022, 18:28

Fuel pump ok?

neil.mccrirrick · 12 July 2022, 19:22

Think so rebuilt it, it will tick over no problem which must mean it’s pulling some fuel. Doesn’t want to rev in neutral easily. Changing the mixture isn’t having much effect but I’m thinking that’s 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.

jeffstevens763@g · 12 July 2022, 20:00

Quote:

Originally Posted by neil.mccrirrick

Think so rebuilt it, it will tick over no problem which must mean it’s pulling some fuel. Doesn’t want to rev in neutral easily. Changing the mixture isn’t having much effect but I’m thinking that’s 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

neil.mccrirrick · 12 July 2022, 20:27

It’s a new diaphragm I do have another. If nothing else I’ll take the carb off again and swap the diaphragm [emoji106]

jeffstevens763@g · 12 July 2022, 20:41

High tension lead or spark plug cap breaking down?

neil.mccrirrick · 12 July 2022, 20:51

I swapped the coils and the leads, top cylinder still fires much better than the lower one

Nos4r2 · 12 July 2022, 21:29

Ultrasonic clean the carbs.

Max... · 12 July 2022, 21:41

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…

A1an · 12 July 2022, 23:21

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.

neil.mccrirrick · 13 July 2022, 08:36

Thanks Alan very much appreciated. I’ve just ordered a cdi that I can return if it doesn’t fix this

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

Very kind re outboard. I’ve a 6hp aux for the rib I’ll pop that on for now [emoji106]

neil.mccrirrick · 13 July 2022, 08:38

Thanks Max I’ll re the thread. I am fed up with it running well in the bin then failing as soon as it’s under load. I’m sure I’ll get there eventually

neil.mccrirrick · 13 July 2022, 08:58

Read the thread thanks max. Plunger is good on mine. I’ve stripped and rebuilt this carb 4 or 5 times now. But only used carb cleaner. Off for an untraditional bath and I’ll swap the cdi out when the part comes.

I’ll update again after that

neil.mccrirrick · 13 July 2022, 08:59

Untraditional??? Ultrasonic lol

Max... · 13 July 2022, 09:13

Best of luck - f * * * i n g OB's!!! Argggggggggggghhhhhhhhhhhhhhhh.

neil.mccrirrick · 14 July 2022, 16:34

Bit of an update. Cdi has not arrived yet and I’ve not took the carb off yet. Boats in the harbour so I thought I’d 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 won’t run and then won’t idle and dies

I still think I’ve 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?

I’ll strip the carb again when I can and look at the primer specifically. It’s got free movement and is returning so looks on from the outside

Summary still perplexed [emoji1]