Yamaha 15hp 2 stroke won't idle

TmMorris · 19 July 2021, 17:57

I've a new to me but 90s 2 stroke 15hp.

I've reconditioned the gearbox and impeller. Compression on both cylinders is 120. In test the engine has been starting and running beautifully.

I managed to use it in anger over the weekend for the first time and it ran well across the Solent and back but in the river it wouldn't idle or run at low rpm. It runs at high rpm.

It doesn't stink of fuel but the plugs are black and oily when I'd be expecting brown and dry ideally?

Going to fit some new plugs tomorrow just to see if that gets it running smoothly again. I'm pretty confident that I don't have too much oil in the fuel mix at 50:1 and my general thinking is that it's running rich.

One question though is should I hear an engine change when holding revs and turning the carb mix screw? I thought that I should be able to feel a change but it doesn't appear to be doing anything?

Fenlander · 19 July 2021, 18:16

The mixture screw will only change running at idle speed.

Sounds to me you need to have the carb off, clean through, check the float height and assemble with new gaskets.

TmMorris · 19 July 2021, 18:58

Quote:

Originally Posted by Fenlander

The mixture screw will only change running at idle speed.

Sounds to me you need to have the carb off, clean through, check the float height and assemble with new gaskets.

Again! Are you suggesting I should have done the carb strip and clean properly the first time?

Fenlander · 19 July 2021, 19:22

Ahh didn't know you'd been there... sorry.

But re the mixture screw then I guess you know at idle around 850rpm screwed one way it should stop the engine... as it will the other way and you normally need halfway between the two +/- a 1/4 turn.

Isn't that happening?

TmMorris · 19 July 2021, 19:50

The problem is keeping the engine from stalling at idle.

I'm hoping that a set of new plugs tomorrow will bodge it back running so I can play with the idle and assure myself it's correct.

If new plugs take it back to running properly then I'll at least be able to reset the mixture, or try.

Then I'm pondering adding some petrol to the tank to dilute down the oil mix and check how that responds.

What's throwing me is that the plugs don't smell of fuel and are greasy so it makes me think there's too much oil in the mix but I'm sure 50:1 is correct plus, I've run 2 strokes before with too much oil and it was just smoky.

At the same time, the fluffing on low rpm and dying is classic fuel starvation as if the jet is a bit blocked.

Carb off will be the next port of call ultimately.

Limecc · 19 July 2021, 20:07

Maybe worth testing the thermostat in case it's running too cool.

TmMorris · 20 July 2021, 07:27

Yup. Good idea. I should have replaced it anyway when I bought the engine so I'm going to order a couple and some fuel filters and change those regardless.

Fenlander · 20 July 2021, 08:59

Re the idle stalling does it cut out in an instant the moment it drops anywhere near idle speed or does it just start to misbehave over a period of several minutes at idle finally fluffing to a stop?

TmMorris · 20 July 2021, 09:21

Quote:

Originally Posted by Fenlander

Re the idle stalling does it cut out in an instant the moment it drops anywhere near idle speed or does it just start to misbehave over a period of several minutes at idle finally fluffing to a stop?

It doesn't cut immediately. It's very much like an old B movie over actor. You know it's going to die but it takes it's time and makes a bit of a song and dance about it.

I've just returned with a pair of new plugs to see what that does. What's good is that it's replicating the issue in the water tank so I feel I stand a chance of resolving. I'm hoping the new plugs get it running properly again and then I can use that position to work back to find the cause.

I'm going to order a thermostat, fuel filter and pump diaphragm kit and replace all of those as I should have done originally.

TmMorris · 20 July 2021, 09:24

I'm also going to do another compression test as the oily plugs could be due to more serious issues which would be a big blow.

TmMorris · 20 July 2021, 09:35

New plugs in. Starts instantly, ticks over smoothly now I've dialled the mix in and wound it back out to a point of smooth running. It revs up well.

Put it in gear and it idles well, any throttle and it dies.

New plugs have a bit of oily residue on them after a few minutes of running.

What seems to be happening is it is struggling in that mid range just off idle and as you are passing to higher rpm. You can feel it in neutral and in gear it's strong enough to stall the engine.

Fenlander · 20 July 2021, 09:52

I have to say this does sound so much like the infuriating and erratic problems that come with carb blockages... particularly when one or more bits of debris floats about to different places in jets and other passages causing changing and intermittent symptoms.

TmMorris · 20 July 2021, 10:14

Quote:

Originally Posted by Fenlander

I have to say this does sound so much like the infuriating and erratic problems that come with carb blockages... particularly when one or more bits of debris floats about to different places in jets and other passages causing changing and intermittent symptoms.

That's one of my schools of thought.

I'm in the middle of ordering a thermostat and diaphragms to rebuild the pump so that I can eliminate those and they should be done anyway given the age of the motor and that it's new to me but I don't see any harm in taking the carb off and cleaning it again.

The problem I had with the 4 a month or two back was identical in symptoms to what I have now this morning with this one in that it ticked over nicely and ran at full throttle well but the pickup was a bit lumpy in neutral and in gear the engine would die. When I stripped that carb I found a tiny grain of grit in the needle which I'm pretty sure was the issue.

Limecc · 20 July 2021, 10:14

Quote:

Originally Posted by TmMorris

New plugs in. Starts instantly, ticks over smoothly now I've dialled the mix in and wound it back out to a point of smooth running. It revs up well.

Put it in gear and it idles well, any throttle and it dies.

New plugs have a bit of oily residue on them after a few minutes of running.

What seems to be happening is it is struggling in that mid range just off idle and as you are passing to higher rpm. You can feel it in neutral and in gear it's strong enough to stall the engine.

If you have no idea of engine hours it might have worn jets but that would cause rich running and this sounds like it dies through lack of fuel.

Did you say the float height is correct?
What happens if you give it a little choke?

My little 3.5/5hp Tohatsu did the same and was fixed when I put the 5hp carb on. Do you have access to an ultrasonic bath?

TmMorris · 20 July 2021, 10:43

Quote:

Originally Posted by Limecc

If you have no idea of engine hours it might have worn jets but that would cause rich running and this sounds like it dies through lack of fuel.

Did you say the float height is correct?
What happens if you give it a little choke?

My little 3.5/5hp Tohatsu did the same and was fixed when I put the 5hp carb on. Do you have access to an ultrasonic bath?

I tried the 'little bit of choke' trick last night with no joy but not again since replacing the plugs. I'll do that now.

My father kept the ultrasonic bath from his shop and it's large enough for the carb to fit. That's a good idea.

I'll probably whip the carb off today and give it another clean.

I don't think this engine has done high hours but I'm pretty sure that it was never properly looked after. It sat on a tender on davitts for years if not all its life.

Limecc · 20 July 2021, 10:54

Rig something up so the tank is higher and hence gravity fed.
Carb might be starved due to an air leak in the line when fuel's pulled under vacuum?

TmMorris · 20 July 2021, 11:47

Quote:

Originally Posted by Limecc

Rig something up so the tank is higher and hence gravity fed.
Carb might be starved due to an air leak in the line when fuel's pulled under vacuum?

Worth trying. I've got the carb off at the moment and I really think it's running rich.

Everything seems very clean and I've not yet found anything to suggest an issue but I can't find the tool for removing the needle which isn't ideal.

ThundercatRob · 20 July 2021, 12:04

check your coil packs.

I recently had an issue with my Tohatsu 50hp 2 stroke where it would idle fine out of gear and rev ok, but bog down and stall as soon as it went into gear.

I put 3 new Naraku coil packs on (after advice from Roflhat on here) and its sorted the issue.

Liam1982 · 20 July 2021, 12:49

I think your engine should run 100-1 not 50-1.

TmMorris · 20 July 2021, 13:40

Quote:

Originally Posted by Liam1982

I think your engine should run 100-1 not 50-1.

I assumed this originally but scouring the Internet seems to be quite adamant that it's 50:1.

I've not found conclusive proof other than the latest ones are 100:1. Mines a 1990 Yamaha model.

Greasy plugs that have fouled and blue smoke had me originally thinking I'd over oiled the fuel.