Yamaha 15hp 2 stroke bogging

[FlyingScot]

What year is your 15? Does yours have a reed valve? Have you replaced the reed valve? It's not a typical symptom but anything A/F related should be eliminated.

Also, the service manual shows how to test the charge coil, ignition coil and the CDI. Have you done these?

[undercutter]

Originally Posted by camerondueck

Yes, that's exactly what it does

Then if that is the case it is definitely a fuel supply problem not a ignition control problem. It is not an air leak as the motor would quit on you.


Remove, inspect and clean all the fuel lines from the connection on the motor to the pump and carburetor. If this doesn't solve your problem then it is likely the fuel pump.

[FlyingScot]

Originally Posted by camerondueck

I'm not sure there's much adjustment possible...is there?

The service manual shows how to measure and the spec. Adjustment is most likely to just replace it. OP states he replaced the float. I'd check it for sure, but a new float likely will be within spec. But definitely check.

[a64pilot]

Originally Posted by camerondueck

Wouldn't pumping the bulb at least temporarily fix this and give the engine a burst of speed? I have pumped it while it runs, no difference.

Then it’s probably not fuel delivery, even a bad pump will be overridden by pumping the bulb

[undercutter]

Originally Posted by a64pilot

Then it’s probably not fuel delivery, even a bad pump will be overridden by pumping the bulb

Well if it isn't a fuel problem then maybe it is a load problem. Bottom end or even a small line wrapped around the shaft?


If it will rev out in neutral how can it be an ignition problem?

[camerondueck]

Remove, inspect and clean all the fuel lines from the connection on the motor to the pump and carburetor. If this doesn't solve your problem then it is likely the fuel pump.[/QUOTE]


I have already replaced the membranes on the fuel pump.

[camerondueck]

Originally Posted by FlyingScot

What year is your 15? Does yours have a reed valve? Have you replaced the reed valve? It's not a typical symptom but anything A/F related should be eliminated.

Also, the service manual shows how to test the charge coil, ignition coil and the CDI. Have you done these?

There are reed valves in the pump, yes.

[Thumbs Up]

You said that you replaced the jets. But did you? Most rebuild kits do not come with new jets. You could actually be running too rich. It is very common for the main jet to become eroded and pass to much gas.

[a64pilot]

Originally Posted by undercutter

If it will rev out in neutral how can it be an ignition problem?

A weak spark can allow an engine not under load to rev, but I don’t think it’s a weak spark, that usually “busts” up under load, which means missing, and he isn’t seeing that.

This is a two stroke, which means if it’s so rich it won’t rev, it will have a very dark spark plug and be smoking like a train, which he hasn’t said it’s doing, plug may even be wet.

Reed valves are on the induction side and exist to prevent reversion of the charge, they are easy to inspect.

Plus if it has the old style of spark advance, then the actual magneto is turned by the throttle and that is easy to inspect with the cover off, engine doesn’t even have to be running.
I’ve never seen that as a problem, but it does fit the symptoms, as does a broken reed valve, but then so does a bad crankcase seal.

I think it’s leaning out, a plug reading should confirm that, one done with a new plug after an extended full throttle run of a couple of minutes, if it’s white then it’s lean. Sooty, then it’s rich.

This is a two cylinder, do the plugs look identical in color?

[Saskatchewan]

As mentioned before try SeaFoam fuel additive, it has performed miracles on several old engines I use.

[nhschneider]

If you’re using fuel that has ethanol added to it, then I would be highly suspicious of the carburetor being fouled, despite the fact that you can see through the jets. When I had a similar problem, I bought an ultrasonic cleaner, disassembled the carburetor, put boiling water into the u/s cleaner, ‘cooked’ it for half an hour and then used an aerosol carb cleaner on all the passages. Solved my problem.

Good luck, fair winds and calm seas.

[undercutter]

Originally Posted by nhschneider

If you’re using fuel that has ethanol added to it, then I would be highly suspicious of the carburetor being fouled, despite the fact that you can see through the jets. When I had a similar problem, I bought an ultrasonic cleaner, disassembled the carburetor, put boiling water into the u/s cleaner, ‘cooked’ it for half an hour and then used an aerosol carb cleaner on all the passages. Solved my problem.

Good luck, fair winds and calm seas.

And then add an injector cleaner to your fuel every time you fill the jerry can to keep it trouble free.

[sterlinghuff]

Does it smoke when it bogs down? I just had my Yamaha 15 2 stroke worked on for the same issue. There was a very small hole in the diaphragm. It was very difficult to see. We switched out the diaphragm and resolved the issue.

[camerondueck]

Originally Posted by sterlinghuff

Does it smoke when it bogs down?

Yes. But it's a 2 stroke, it smokes all the time

[camerondueck]

Originally Posted by a64pilot

A weak spark can allow an engine not under load to rev, but I don’t think it’s a weak spark, that usually “busts” up under load, which means missing, and he isn’t seeing that.

This is a two stroke, which means if it’s so rich it won’t rev, it will have a very dark spark plug and be smoking like a train, which he hasn’t said it’s doing, plug may even be wet.

Reed valves are on the induction side and exist to prevent reversion of the charge, they are easy to inspect.

Plus if it has the old style of spark advance, then the actual magneto is turned by the throttle and that is easy to inspect with the cover off, engine doesn’t even have to be running.
I’ve never seen that as a problem, but it does fit the symptoms, as does a broken reed valve, but then so does a bad crankcase seal.

I think it’s leaning out, a plug reading should confirm that, one done with a new plug after an extended full throttle run of a couple of minutes, if it’s white then it’s lean. Sooty, then it’s rich.

This is a two cylinder, do the plugs look identical in color?

Thanks for this. I will check these things again. Been focused on other boat projects the last few days, but will get back to the outboard soon.