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2015 Honda BF115D

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nanserbe

New member
First time poster here.

I have a 2015 Honda BF115D.

During operation, the engine feels like it has a flat spot in the power band when getting up on plane. Probably around 2500-4000rpm If you push past it by accelerating further it smoothes out and cruises fine.

The engine will not turn past 5900rpms, when it has a prop that should allow it to hit the rev limiter. With a 13P prop it should be able to, as a friends same boat and engine will hit it with a 15P or 16P.

The engine also runs rough when it is in gear at idle (650rpm).

So far I have replaced both of the small filters under the hood in the plastic cups, replaced the spark plugs with NGK (I may go back to Denso to see if that helps),

There are no warning beeps or MIL lights from what I can tell. The engine has under 200 hours.
 

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Hi,
Welcome.

I have no experience with this outboard so these are just thoughts/ideas for you to consider until someone WITH experience answers.

Have you tried pumping the fuel hose primer bulb during a "low power event" to see if that makes a difference? Might need a helper to try that.
Also, pinching off the vacuum line to the fuel pressure regulator (FPR) allows full pressure from pump to enter the fuel rail.
Check the inside of that vacuum line for any sign of gasoline. If it's wet, the FPR is leaking and should be replaced. See items 5 and 7 in link below.


Another thing that comes to mind is the main fuel filter might be getting dirty. Item 40 in link below.


Also, item 12.... cover, water jacket... indicates a fuel cooler. I have never disassembled a VST but have read about the coolers getting plugged on different models. Not sure if that would be the case with yours but just tossing it out there.

There are other things but I'll leave it at that for now and mention those if you don't find a quick solution.

I've always had better experience with NGK plugs on Honda but if she ran better with the Densos, it's hard to argue with trying a new set of those.

Wishing you good luck.
 
Start with basics, compression test, cylinder drop test, sounds like it's running on 3 cylinders. Also check fuel pressure while under way.
 
I pulled all the plugs out to see if I could tell if one was different than the others, these NGK plugs have about 50 hours on them.

I have a set of Denso plugs to go in and see if that changes anything, I’ll run a compression test when I change them out.

Another thought was the oxygen sensor possibly being faulty?

I’m out of the country right now traveling so I’ll do all this when I’m home.
 
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It's expensive to guess, oxygen sensor needs to be looked at with diagnostics to see in it's operating correctly, they are not really a problem on these motors. I'm thinking low compression on one cylinder given the symptoms you describe
 
Back from vacation, I will compression test this week and post back.

Some more backstory, the dealer I bought the boat from replaced the high pressure fuel pump right before I bought the boat. He attributed it to the boat sitting for years before being sold to me.

The second time I went out on the boat the float in the VST fell off and vented fuel out the vent hose.
Pretty crazy, but an easy fix:
IMG_3829.JPEG


Later on, before I redid some of the boat's fuel system (tank clean and new racor filter) I found a little water in the VST fuel when I drained it:
29.10.2025_18.12.25_REC.png

I also replaced both filters in the engine (VST and the preVST filter) when I drained the VST. They were both pretty dirty if I recall, almost with a clay-like substance. I might do them again soon and drain the VST once more.
 
Also, only because I haven't redone them yet, I bought all new fuel line (3/8" Shields 368), and a new primer bulb.

I grabbed a couple 12oz of bottles of Ringfree. My plan regardless of the compression is to run a shock treatment on it.
3 gallons of ethanol free and 10oz of ring free is about 40:1 ratio, maybe more like 38:1.
 
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Next up, plugs. I replaced them with a set of Denso to see if they're any better/different. I know Denso isn't the favorite flavor here, it's just what I was able to get locally.

These plugs had about 25-50 hours on them.

In order from top to bottom:
IMG_9806.JPEGIMG_9807.JPEGIMG_9810.JPEG
 
Something worth noting here - I noticed that the NGK plugs may be the wrong ones.

The manual as well as boats.net call for ZFR6K-11, while I had BKR6E-11.
All the specs look similar (heat range, length, projected tip, etc.) but with two different part numbers google says that there may be a difference in how far the electrode projects out. I'm not sure if this will make any actual difference, but I'm trying out a set of plugs that were listed in the boats.net site as well as the BF115D manual (Denso KJ20DR-M11).

What NGK plugs are people usually using in these engines?
 
Started the boat up today during a break in the PNW rain.

Revved it to around 2500rpm and noticed that there is an occasional miss.

Videos here:
 
iang6766 said:
Compression on a healthy motor should be above 180psi, usually 210. What valve clesrance are you running?
Click to expand...
Whatever the factory setting was, there’s 195hrs on this engine and I haven’t messed with the valve lash at all.

Correct me if I’m wrong here though, shouldn’t the PSI = atmospheric pressure x the compression ratio?

So for a 9.6:1 Compression Ratio on this BF115D x 14.7psi (sea level atmospheric pressure) it should be about 141psi at a minimum.

My compression test was with all plugs out, throttle body closed, injectors unplugged, and the engine was cold.
 
I've comp tested lots of these engines, some up around 2000 hrs, and they all return around 200 . Your gauge may be incorrect. Are you chasing a problem?
 
iang6766 said:
I've comp tested lots of these engines, some up around 2000 hrs, and they all return around 200 . Your gauge may be incorrect. Are you chasing a problem?
Click to expand...
It very well could be inaccurate, it’s one of those Maddox ones from Harbor Freight.

The problem I’m running into is not being able to hit the rev limiter with two props that should absolutely be able to (13p &15p) and it’s developed a slight random miss at idle and under load. (See the above videos)

Personally I think it’s a clogged injector(s) not atomizing well. Because of this I am considering taking the bowl off my water separator and fill it halfway up with injector cleaner and run it to clean them out today.

I’m also going to put it in the water in a little while and run 3 gallons of 40:1 ring free through it.
 
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