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Honda BF150 surges at 3500-4000 rpm

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sjlaster

New member
I have twin Honda 150s that are 2015 motors with 900 hours. The Port engine runs perfectly. The starboard engine runs great except that when both engines are set to around 3500 rpm the starboard engine will surge up and down from 3200 - 4000 rpm. When I shutdown the port engine and run just on the starboard engine, it does not surge. If I drop the port engine down to 2500 rpm and run the starboard engine at 3200-4000 the starboard engine does not surge. When rev up starboard engine out of gear it does not surge. I have tried

1. Different trim angles - no change
2. Adjusted the steering angle of the two engines - no change
3. Removed the transom transducer located on the starboard side - no change
4. temporarily disengaged the Air Intake Bypass so it is always working on the high speed runner - no change
5. I have new/clean filters
6. I have good fuel and fuel lines

At any other rpm the engine runs flawlessly. I welcome any thoughts.
 
Hi sjlaster,

That IS a pretty strange symptom indeed!

At first, I thought maybe a bad MAP sensor or fuel issue. But, after digesting the fact that it runs fine pretty much by itself, it's certainly got me stumped as to what might be causing this.

Did this come on gradually or suddenly?

When you say that "filters are clean" does that mean you opened up the VST and cleaned it? I know the float and needle valve can stick in there sometimes.

But this... sort of... sounds like it might be a loose connection that is being "buzzed" by the very specific harmonic vibration set up with both motors at that particular throttle setting. Where that would be is the question. But you might want to look at ECM connector and maybe crankshaft sensor to start.

Actually, I would wiggle and gently tug on every electrical connector I could easily get at. Fuel injector connectors have a tendency to get loose via vibration. Ignition coils too.

This is likely going to be a very difficult puzzle to solve without unfettered access to a Dr.H and driving but, sometimes anyway, a little snooping around the electrical system and a can of contact cleaner can pay off.

There are guys here much sharper than me that you will hopefully hear from soon. Just know that we're out here rooting you on to find a solution.

Good luck.
 
Hi jgmo,

Thank you for taking the time to reply and your thoughts. I agree this is a tough one to figure out. I think checking the connections is good advice. I also agree that in the end it may require a lot of testing/time with Dr. H.

I welcome other thoughts.
 
They are not fly by wire they are mechanical shift and throttle controls. The engines have analog gauges and are NMEA connected to a GARMIN. The NMEA values match the analog values.
 
When you decrease port to 2500 I assume you are increasing the stbd throttle to regain 3500 which must have dropped, so the observation here is that you have the 3500 issue with a specific and narrow throttle lever angle setting only. You will need Dr H or a diagnostic tool to check for errors. I would also specially check the Throttle Body anngle sensor voltage reading at idle and WOT (no meed to have the engine running, but you do need to have the ECM energized). Also check throttle body for corrosion at butterfly/venturi.
 
It sounds more like a fuel supply issue.Hence the motors runs fine on its own,but when both motors are running , the one is starving the other with fuel.Just athought.I would start there ,with fuel delivery first.If the outboard that is surging and does not give you a code error. As mentioned before it could be TPS issue as well ,if not operating smooth across throtle settings,but it should be consistent ,whether on its own or together. Dr H will help in checking that . my assumption is that when the load is shared it is affecting the problematic outboard and not when on its own. Hence the fuel issue opinion,just my 2 c.Hope it helps.
 
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