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2018 Honda 40hp outboard won't reach peak rpms

T

troutdock

New member
When throttling up, engine runs strong, boat pops up on plane. But when trying to achieve max rpms, engine suddenly retards itself. It does not die, but idles poorly til engine is throttled down.
Fuel filter is clean, no water visible. Screen is clean. Emptied tank, filled with fresh 87 octane fuel, no ethanol. Engine starts without hesitation. Low idle fine. Fast idle fine. Acceleration strong til last 10%.Failure repeats.
 
Hi,

I've not worked on one of these. I'm assuming it's a fuel injected BF40D model?
Approximately how many hours on your outboard?
Have the spark plugs ever been changed or valve adjustment checked?

Mostly I'm answering to let you know that people are seeing your post and many are interested. Hopefully someone else here (like ian6766) has the knowledge you need and will chime in soon.

I think that this symptom could point to any of several things. From inadequate fuel flow pressure/volume, faulty pulser, 02 sensor, or even a dirty throttle body.

I don't know if Honda has a diagnostic tool for these. But, if they do, you may need to seek help from a shop that has one in order to accurately diagnose this.

The authorized shop manual, in case you want one, is available at the link below along with some free download PDF's for the 40hp model as well as many others.

Honda Marine | Shop Manuals Publications | Honda Power Products Support Publications

publications.powerequipment.honda.com publications.powerequipment.honda.com

Good luck.
 
I’m not familiar with this ob either but I’d ask if you’ve owned it a long time and this problem is new?

I’d run a high dose of Sea Foam in the fuel and run it at high rpm under load and see if after an hour or so of this anything changes. But check your spark plugs first to see if they are clean or fouled in some way. Do the easy stuff first.
 
Since it's a 2018 BF40 with PGM-FI, you've got electronic fuel injection on that engine. Your troubleshooting so far has been solid. The symptom you're describing, strong everywhere except the last 10% of throttle, points to a possible fuel delivery issue under peak demand.

On the EFI side, I'd want to know what your fuel pressure looks like under load at WOT, not just at idle. A weak fuel pump or a partially restricted line between the VST and the rail can deliver fine until the engine needs max flow.

Injectors are worth looking at too. They can build up deposits where they flow enough for cruise RPM but can't keep up at WOT. A fuel injector flow test shows that pretty clearly.

Has anyone looked at the injectors on it?
 
Wow,
I'm awed by all of you that are trying to help me. Thank you.
I'm not the original owner, but this engine has very few hours. I'm inclined to think fuel pump or injectors. I fear it's time to get a pro involved.
 
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