Mercury 15hp 2 Stroke

[neil.mccrirrick]

Hi all. Have a headache with the carburettor on this engine. Symptoms are it bogs down when throttle is applied and dies

I’ve stripped the carb and fuel pump. Replaced all gaskets snd rebuilt.

When you pump the bulb I can see fuel passing into the carb body from the tube that connects to the main jet. It will eventually stop when the bulb gets hard which I guess is the float rising enough to stop the fuel demand

I’m guessing there is wear on the main jet threads and it’s allowing fuel to pass where it shouldn’t? But I am guessing can anybody advise?

Thanks

Neil

[Fenlander]

In my experience with these portable 2-strokes bogging down has been much more associated with a blocked jet or fuel shortage rather than flooding. It's surprising how hard it can be to clear some carb issues.

The other thing which to me is less likely is a big air leak. I did once have a hose split on the bottom of the carb flange on this model, you could check that and other sources of an air leak.

I assume you have tried raising the revs really slowly to check it isn't a lean sneeze type weak mixture in transition when throttling up... I'm wondering about idle mixture being a bit weak and possibly an accelerator pump problem if that were the case.

[TmMorris]

I'd agree with Fen.

My first thought is that you've got a bit of dirt in the needle. Enough fuel gets through to idle well but you then get starvation when you open the throttle.

Can you fudge it to run on throttle by pulling the choke slowly out to the point where you're restricting the air enough to match the amount of restricted fuel flow?

I've had similar with an air leak but that was a bolt holding the top plate on the carb that had come loose but almost every time it's a tiny bit of dirt in the needle that's been the cause.

[A1an]

Am I right in thinking you are in easdale neil?

[neil.mccrirrick]

I’ll strip it again. I’ve drowned it in carb cleaner it looked spotless but I appreciate there can be a minute bit of grit and that can be it

Once it starts I can rev it in neutral in the tank, but then it will start to fade and die. This tells me it could be an air leak and the pumps not pulling, which was why I changed all the gaskets and stripped the fuel pump.

Checked the hose under the carb. Originally that did have a gash in it but I cut that back so all good there now.

[neil.mccrirrick]

I’ll take the fuel pump apart again and check the seals etc

[TmMorris]

Originally Posted by neil.mccrirrick

I’ll strip it again. I’ve drowned it in carb cleaner it looked spotless but I appreciate there can be a minute bit of grit and that can be it

Once it starts I can rev it in neutral in the tank, but then it will start to fade and die. This tells me it could be an air leak and the pumps not pulling, which was why I changed all the gaskets and stripped the fuel pump.

Checked the hose under the carb. Originally that did have a gash in it but I cut that back so all good there now.

Could be losing vacuum. Is the pump the usual little diaphragm box on the side of the carb? Could also be a blockage from tank to pump? Does the engine come back to life when you squeeze the fuel primer bulb?

Did you blow some air through the needle when cleaning? I tend to disassemble a carb in a pot of petrol, scrub everything with a tooth brush and leave overnight but take the jet out and blast carb cleaner through it and visually check.

[neil.mccrirrick]

It is the little pump on the side. As it dies if I ores the bulb it floods I can see the petrol entering the body of the carb

Yep blew through everything it looked spotless. I’ll strip again and see if anything jumps out

[TmMorris]

When it starts to die, does bringing it back to idle settle it down and then after a while you can rev it a bit again before it then starts to fade once more?

[neil.mccrirrick]

No it just dies then. Pump the bulb petrol visibly pumping into the body of the carb. Bulb goes hard and petrol stops spraying into the carb. Engine will start

[TmMorris]

Can you keep the infine running by squeezing the bulb?

Does it idle permanently or does it eventually use up the fuel in the float chamber and die?

[neil.mccrirrick]

Squeezing the bulb will kill it. I’m going to strip it down now and see if something is maybe not seated somewhere [emoji106]

[TmMorris]

Float. Not floating?

In theory, squeezing the bulb should put fuel into the float chamber until the float closes off the route into the carb. If able to flood the engine by squeezing the fuel bulb then that suggests the float isn't closing the entry?

Not sure why that would cause it to flood on revving but maybe the float valve also regulated how much the pump hurls in and maybe the pump ends up forcing fuel through?

[neil.mccrirrick]

All fixed and I have to confess to user error [emoji33]. Bottom gasket on the fuel pump there are two that are almost identical. Both fit however one has a small bend in it to allow fuel to flow.

I’d not seen it and fitted the wrong one as I changed all the gaskets out. I kept the old and tie a side by side comparison and found it. Re built now and ticking over fine. I’ll need to get it on the water later this week to test under load but I think all is well now.

Thanks both. Most helpful and super responsive. Hope I can help you at some point

Best

Neil

[TmMorris]

Excellent. Enjoy.

[Fenlander]

Well done, fingers crossed for the outing. A daft self imposed error found in reasonable time is way better than a whole season of imperfect running.

[neil.mccrirrick]

Ain’t that the truth [emoji1][emoji106]

[neil.mccrirrick]

Not resolved this yet [emoji35]. Ticked over and ran at wot then died.

When it dies the bulb is still hard which makes me think it must still be pulling air from somewhere.

The only place I had doubt is the cover that is marked top. There is a cut out here which matches with the gasket however when the the cover is tight there is a gap between the cover and carb body by design. Seems to me this will let air in?

[Fenlander]

Did you mean that post to have an image?

[Fenlander]

Look at this thread of mine from 2015, I assume the same carb? If so does anything in those posts or images prompt any thoughts?

https://www.rib.net/forum/f36/marine...aul-67565.html