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Honda BF50 bogs down, expert mechanic perplexed. What say you masters of mechanics?

I have a Honda BF50 that I purchased mid last season from a dealership. It is missing it’s serial number plate, but the dealership thought it was a 2000 year model. When it was fired up at the dealership, it ran nicely. Compression test was perfect. However, once I got it on the water, it was perpetually disappointing. Hard starting (sometimes VERY hard to start), sometimes runs rough, other times runs nicely. But, it has always bogged down when accelerating rapidly.

So, I took it to a highly respected mechanic shop. Amazing reviews, great reputation. Local guides choose this guy, etc. I told the mechanic I was hoping this would be a motor I’d keep for a long time, so that I wasn’t concerned about the cost to get it in tip-top shape. The mechanic has been great. Kept me up to date on progress. Replaced some gaskets, got the carbs spotless, valves adjusted, carbs synchronized, and more. It now runs like a Honda should run (starts immediately, runs silky smooth) except for one thing—it still bogs down under rapid acceleration. He has some tool that allows him to run it under load in the tank.

He still has the motor, and hasn’t cried uncle, but he’s close. He just can’t even come up with a hypothesis about what’s causing the bogging down. He’s had it in the shop off to the side while working on other outboards, and every once in a while an idea pops into his head, but the idea never comes to fruition.

I asked him to not give up yet, and that while he’s noodling on it, I’d post to a few forums to see if any Michael Jordan of Honda Outboards mechanics had any ideas. I’m not sufficiently mechanically skilled to try to re-state all he’s tried. But, assuming he’s a very competent mechanic (which I’m confident that he is), here is my question: Do any of you Jordans out there have any advice? Anything he should consider that is decidedly non-obvious, even to an expert mechanic who has spent hours troubleshooting? Anything specific to that vintage Honda?

Thanks ahead of time for any ideas!
 

jgmo

Silver Medal Contributor
Well, no MJ here, but as I read your post, a few things came to mind....
#1...fuel starvation under heavy load. If it starts nicely and idles smoothly and purrs "as a Honda should" but still stumbles on hard accell then it could very well be a lack of the fuel volume it needs p to provide max power.
What might cause that?
Well, HONDA carbs are FAMOUS for being very difficult to get properly clean once "mineralized". Not impossible by any means but difficult. They are very finicky with super tiny passages that demand full and complete attention of anyone attempting to clean them. They also have a low speed emulsion tube they call the "jet-set" that is hard to next to impossible to clean and that item has a tendency to develop a crack
Notice I said "low speed" emulsion tube. But these carbs REQUIRE that ALL circuits contribute fuel during high speed operation to attain full power. That includes the idle circuit.
That's why I wouldn't trust ANYONE (except maybe Alan Hicks) to "rebuild" a Honda carb on my behalf and that's why I discuss them first.

Other lack of fuel causes might be a weak fuel pump simply not providing enough volume or an issue with the fuel tank or fuel lines. Vibration is a bit€h and can play havoc with the tank pick up tube or even the fuel hose quick disconnect fittings.

#2...a bad ignition coil that is breaking down under the stress of high speed operation. Start losing a coil on a 3 cylinder engine and the power won't be there. Just because a coil "ohms out" on the numbers doesn't mean it's not defective. The problem is testing for it ot on the water or without an oscilloscope.

#3....Weak valve spring. A weak spring (or more) will not have the valve(s) closing correctly at higher rpm.A weak valve spring is likely NOT to show up on a conventional or "static" compression test. It might on a "running" compression test but doing one on a 3 cylinder might not work so great. Don't know, never tried it. About all that could be done there is to either compare free heights or put 'em on a tester. It never hurts just to pull the valve cover and eyeball the valve train in a case like this.

...what if it's not the outboard?
I mean, is the outboard mounted correctly? Too deep and you get drag and possibly exhaust restriction. Rare but possible. Also, getting back to "bad vibrations", higher rpms/speed generally results in unique harmonics that can cause a bad wire or loose connector to become a point of high resistance that might cause an issue. This could apply to a bad harness to a wonky kill switch.

Just some musings from a NON All-Star that can't (and never could) dunk.

Good luck.
 

Alan Hicks

Advanced Contributor
Here's the reply I posted to this same user's question on another site -

"The hard starting is often a sign that it's lean - especially when cold. They can be a bugger to keep running til warmed. A bog when accelerated could easily be the same - a sign it's lean. This leaves me wondering about a few things. Like where are the idle mixture screws set, exactly (the "factory" suggested settings often leave the motor lean)?

Have all 3 of the tiny O-rings that come with the carb kits been replaced on all 3 carbs? If one (or more) of these is cracked or hardened, they can easily cause a lean condition. If this engine is still equipped with the "tamper proof" caps on the idle mixture screws, that's a good sign one of these tiny o-rings have NOT been replaced on each carb where the caps are still installed.....

Has the motor been checked for vacuum leaks by spraying something flammable in the area where the intake manifold bolts to the block with the engine running, where the carbs bolt to the manifold, and also the condition of the vacuum lines. Is there a possibility one of those is cracked or improperly plumbed?

Have the carbs been synchronized so each is pulling the same load/amount of air?
 
Thanks, both of you for your thoughts and advice! It's great to have Scotty Pippin and MJ so accessable. More valuable in my mind than tossing a ball through a hoop :cool:
 

cbs

New member
I have a similar issue with my bf15 runs great, starts easily, but on quick acceleration it will stall. But if I slowly accelerate it runs fine. Replaced filters cleaned carburetor even put carburetor in an ultrasonic parts cleaner and still have same issue. The motor seems to be running rich, lots of exhaust. I believe the motor is a 2006 4stroke.
 

jgmo

Silver Medal Contributor
cbs,,
Welcome new member!

This is a pretty old thread for a multi-carbed, 3 cylinder outboard.
If you want to reach other BF15 owners that may have solved a similar problem to yours then you should start your own thread. Thread hi-jacking not recommended anyway.

These carbs are hard to clean and in a case where the owner complains of poor acceleration I would usually urge more cleaning with particular attention paid to the accelerator pump circuit and test of the accelerator pump spray orifice.

But...you say it's running rich. That's a bit of a curveball since these carbs usually have the effect of running lean when they start plugging up.

The running rich has me thinking this could be something other than carb.

First thing I suggest is to check engine timing.

When was the last time you replaced the spark plugs?
If they're fouled then it could be something as simple as that. I would check them but not clean them. Just replace if fouled. Cheap solution/insurance. Make certain they are properly gapped prior to install.

Other things that come to mind are valve lash adjustment and pulsar coil connectors or wiring.

Good luck.
 
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