I've been dealing with this issue for about a year so I'll list the symptoms, what came before and what I've done about it so far (and my test results).

Boat began to bog down around 3K RPMs and would struggle to get on plane. The engine is not overheating, engine doesn't appear to be going into idle mode. I took it to a marine mechanic who rebuilt the carbs and did some basic maintenance. The engine started easier and sounded better after but still bogged down under load above ~3K RPMs.


What I've done so far.

1. Drained the tank (myself)
2. Replaced the fuel line from antisiphon valve at tank to under-cowling filter bowl (myself)
3. Replaced the external Yamaha 10u water separator/filter (myself) (found a lot of [censored] in the filter when I poured the gas into a clear container)
4. Replaced the primer bulb (myself)
5. Replaced the under cowling filter assembly (myself)
6. Replaced both fuel pumps (myself)
7. New plugs (myself)
8. Compression tested good on all cylinders (tested myself and done again by marine mechanic)
9, Spark confirmed on all 6 plugs, not gap tested or tested at high RPMs
10 all 3 Carbs were rebuilt (by marine mechanic)
11. Stator and trigger resistances in spec

Likely unrelated but still..

11 Water pump has been replaced ( by marine mechanic just because it was time)
12 Thermostats replaced (by me, they looked like they needed it)
13. Tested the temp sensors to ensure they work. (myself)

None of this corrected the symptoms


However, ran on separate tank today with new gas and new line to the inline filter assembly (under cowling) and the problem persists. So I've ruled out the rest of the fuel system from there back to the fuel pickup at the below deck tank. .I could have done that earlier but many of these things I replaced actually needed it and I felt confident something in the fuel path was causing the issue.

What hasn't been done

1. Timing checked (not sure how it would change by sitting there but at this point I am up for anything)
2. CDI checked or replaced (not familiar enough with this engine to even know if it makes sense)
3. Smaller fuel line jungle at carbs replaced (no signs of leaks)
4. Stator voltages not yet tested with DVA

So basically the outboard will feel powerful and push the boat until the RPMs under load gets to about 3300. At one point I saw 4100RPMs but it quickly bogged down to around 3100 and there it remained. It runs fairly decent at that RPM, no odd noises or misfiring.

Primer stays firm when positioned properly (vertical) and there are no obvious leaks (on two separate fuel system tests, one using the outboard tank/fuel circuit and one using the below deck tank and fuel circuit).

I haven't ruled out the carbs and the possibility that the rebuild did not address the underlying issue with one or more of them.

I am open to suggestions.

Thanks in advance.

I'm not a master certified tech, but my diagnostic steps would be:
Pull cowling and carb covers. At the ramp if possible with motor in gear and clear of obstruction, look into each carb throat and jab the throttle. You should see the fuel spray in each carb throat. If not, the one that's not spraying is the culprit.

Check your plugs, does one look wet or clean? Coil or power pack.

Then look for any burnt, corroded or loose wires.

Thanks Pat.

I planned on inspecting the spray when I can get someone to accompany me on the lake. I was also going to spray carb cleaner down the jets. Once it gets ironed out it should be all caught up on the ta-do list.

If you don’t already have a good water separating filter they are essential with the garbage gas we get now.

Has the stator been checked out? I’d be looking there if it hasn’t.

Also, a much easier way to troubleshoot the carbs is to hit your choke/primer when the motor begins to bog. If it takes off and runs better, then yes it’s a carb/fuel delivery issue. But if it runs even worse, then you are looking at an ignition issue. And just because it has spark at idle, doesn’t mean it has spark throughout the entire RPM range.

Yes, the Yamaha 10 micron fuel/water separating filter. I replace it regularly and check the contents by pouring the gas inside into a clear container. Didn't find any water but did find enough debris inside to make me think the fuel hose between the tank and the separator was breaking down, so I replaced the hose and ran for a while and checked again with no debris in separator.

No, you need to add a water separator filter kit. Let it do the heavy lifting, your motor filter can't keep up with the gunky fuel. EVERYONE should have one, it's cheap insurance.

Actually using this with this



The separator itself is new. The base is pristine.

This is obviously external to the cowling while the under-cowling filter assembly is also new (and in use).

I haven't checked the stator myself. I need to see the instructions for checking resistance while off and AC while running, where to measure on which Deutsch connections

Choking doesn't improve the performance.

I’d be looking at that stator for sure then. I replaced one on my Dad’s old 03’ 225 VMAX, very similar symptoms.

Does your motor have an O2 sensor? That might be a possibility too. I replaced one of those on his motor as well.

And a side note on checking stators, they can sometimes checkout just fine when cool and not in operation, but become faulty under hot running conditions.

Ok, great.
It wasn't clear.

I don't know about the O2 sensor.


The symptoms happen right away, just out of the no wake zone while revving up to get on plane. They don't get better or worse based on having the engine off for periods of time on the lake and starting again.

Pat,

My plugs are all wet, all look about the same. They aren't sloppy wet, just wet. A couple are slightly more fouled than the others but nothing seems significant considering how much idle running I've done testing.