Yamaha 15 hp 2 Stroke Bogging - Diagnosis Please!

[TJ11]

Hello fellow boating crazies! Happy 4th to you all. I am bit frustrated at the moment trying to diagnose an issue with my normally bulletproof 2018 Yamaha 2 stroke outboard 15FMH. Any help or suggested next steps are most welcome! 100 bonus points to the winner. Thank you in advance!



The Issue: The motor starts, idles, and even after warm, bogs down or rather struggles to ramp up when you jump on the gas. It doesn't sound right at this stage, and feels like 1/2 the engine it was before with no top end. No ability to plane despite only one person with a 10 foot RIB. Before it would jump on top of the water ready to drag race.



Steps Taken So Far to No Avail:


1. Verified tight fuel connections and no leaking fuel anywhere.

2. Verified open vent. Tried running without the fuel cap on for maximum vent, no improvement.
3. Compression on both cylinders is 120psi.
4. Verified spark. Put in new plugs anyway with the proper gap. Verified spark again.
5. Replaced all of the fuel with fresh premium. No improvement. Added Seafoam treatment to fuel, no improvement.
6. Cleaned the carb (I promise it's clean). Air compressor to blow carb clean, carb cleaner, removed the jets, see light through all jet openings, ran wire through them, etc.

7. Primer bulb is tight when running and not collapsed. Tried pumping the bulb while under load trying to get on a plane and perhaps verify a faulty fuel pump, no improvement.

8. Opening the choke when under load kills the engine.

9. Fuel filter looks clean. Tried bypassing the fuel filter anyway, no improvement.
10. Ran a clear tube directly from the tank to the fuel inlet on the carb/fuel pump to bypass and see if the tank pickup or fuel hose or bulb or fittings were the culprit. Could see the fuel moving through the clear tubing. No improvement.
11. Rebuilt the fuel pump as suggested by the local Yamaha dealer (replaced the gasket and both diaphragm pieces). No improvement.
12. Verified the pick up tube in the tank is intact, not ruptured, and the pickup screen filter is clean.
13. Verified the air inlet housing attached to the carb is clean / free of an obstruction or nest, etc. This engine model does not have an air filter.
14. Verified nothing is tangled around the prop or shaft.



Next up, I will replace the 6 year old fuel hose, bulb and fuel fittings, although I do not think this will solve anything due to the test with step 10 above.


What am I missing!!!??????!!


Thank you.

[chris mac]

All I can offer is a situation that came up last year. Our friend had similar problems, and long story short... It was a bow line(that wasn't long enough to hit the prop) was in the water. It came just close enough that it interrupted the water flow leading to the prop. Once he pulled it aboard, his problems went away.
So I suggest looking to make sure there is nothing underneath interfering with water flow

[JPA Cate]

I'd re-do the carburetor, but that said, if the fuel hose and bulb are old, they could have delivered a particle that has now clogged a jet, so that replacement might be in order anyway. My suspicion is that something got in the carb after you got it pristine.

Ann

[Loredo]

Since you ask what you could be missing.

Mine is a 2000 something 15Hp two stroke, thus I don't know how similar (or not) our engines are. I fought with a very similar problem to yours some years ago. I did +/- what you've done to solve that mystery.I make this long story short... somehow I convinced myself that the Yamaha needed a 2% oil mixture. That's what I did, 2%. Turns out that my engine requires a 1% oil/fuel ratio. OPS! A mayor brain-fart...Sometimes we do very stupid things for no apparent reason! Once I returned to 1% the world was perfect again. Dunno, maybe?

[ejs]

Carbys for these are about $25 on Aliexpress. I've battled cleaning them before (there are many tiny passages that need to be perfect) but now I just keep a spare and swap them out.

[Sparx]

Check the ignition coil.

Your troubleshooting so far has eliminated many possibilities. Good choices you have made. Next step is to check the ignition coil. The symptoms you describe of bogging down are classic ignition coil fail. Likely your engine is running on a single spark plug or a pair of low energy sparks.

First check that the spark plug wires are righteously connected to the spark plugs. Inside the spark plug boot is a wire clip that is supposed to encircle the anode on the spark plug, which I have seen go akimbo resulting in a poor connection. Also check the low voltage wires supplying power to the coil to confirm good connection and no shorts. Follow the wires back to their source to insure they are not pinched or scorched. If all good put an amp meter on each spark plug wire. Should be a similar power to each plug.

A new coil for that engine is not an expensive item to try swapping out. Well with in your skill set.

[TJ11]

Thank you for the suggestions. Still trying to figure it out. I have always used premium fuel with Yamalube oil at a 50/1 ratio per the manual so not sure I should mess with that. Since my last post I have cleaned the carb again and replaced the fuel line and primer bulb without improvement. Verified again that the spark plug connections are clean and tight including the wire clips. I went ahead and replaced the CDI Unit with another Yamaha OEM unit with the understanding that sometimes these can provide spark but start to fail at higher RPMs. No improvement. Verified all of the CDI and ground connections are clean and tight. Tested the resistance on each coil, and they are both about 2.2 kilo-ohms, so one is not obviously bad to me. I suppose the replacing the coils is next. Still stumped. Thanks again

[CarlF]

I assume this engine has some years on it. Sometimes you just need new parts.

I'd replace the coil and all related wires.

If that doesn't work, I'd replace the carburator. I know you cleaned it but carburators sometimes just reach their end of life.

[Sparx]

TJ11, thanks for the update. I have wondering how you were progressing. At this point you have good information about what is NOT the cause.

CarlF, I have experienced exactly what you are talking about. I cleaned and rebuilt the carb on my Yamaha two stroke many times, but eventually the choke shaft became so corroded it would not turn easily enough to operate as required. That was the signal I needed to replace the carb.

[jamhass]

How about the timing? Had an outboard once that had similar issues. Turned out that the flywheel had spun a bit (long story involving a dinghy flip) disturbing the timing.

[TJ11]

Resolved! Quick update to help those who might stumble upon this post. I cleaned the carb and and jets meticulously for a 3rd time and soaked them for a couple of hours in Chem-Dip carb cleaner. No improvement. Finally took the advice from the helpful posts above, swapped in a new $50 Chinese non-Yamaha carb, and bam! back in business. Something wore out I suppose in the old carb that confuses me, but going forward I am going to just have a spare knock-off carb on hand for when a standard carb cleaning does not set things right. Thanks!