BF100 stalls at WOT

Hi all,

I have a 80's Honda BF100 long shaft used on a 22' sailboat that has a stalling problem that has me stumped.

Here is the issue : The engine starts fine (cold with choke, warm without) and will idle fine for extended periods. It will also run at ~50% throttle with no issues. The problem is that when you give it close to WOT it will run great for maybe 10 seconds then bog down to idle for ~1-2 seconds before stalling. Once it has stalled this way it is very hard to get started again, choke or not. You have to leave it for a few hours / days and then it will start again fine. Rince and repeat...

Here is what I have done for now : When it stalls I checked if it was getting enough fuel and there was fuel in the carb bowl (pours out if you open the drain). I took the whole carb appart and cleaned it, put it back together and readjusted as per the manual's instructions. Checked if the fuel pump was giving good pressure and it shoots out fuel if you disconnect the line from the carburetor while the engine is running. This leads me to think it is not a fuel delivery problem... Can the carb be giving too much fuel at WOT and basically flood the engine ? That would explain why it is so hard to start after...

Any other ideas? This has me stumped!!! Thanks!

I know absolutely nothing about that engine, but from what you described it sounds like you may be overheating and triggering the automatic shutdown.

Kinda has me stumped too.
I'll say up front, I have questions but no answers

Have you tried running it at full throttle with the hood off? These old girls have a bad habit of developing an exhaust leak in the oil case that can starve the under hood area of oxygen. No oxygen, no fire. If that's what's happening it should run at WOT all day with the hood off

Could it be overheating? With the hood off, you should be able to touch the side of the cylinder head for a couple of seconds without being burned. An infrared temperature gun is more "scientific" but the old burned finger style works too.

If it lives on the transom, when was the last pump and thermostat services? The impeller can pump water with broken/ missing vanes but still not provide enough flow to cool the engine properly. Same applies to a partially open stuck stat.

You indicate that you have a manual so have you checked the timing?
I would also check valve clearance. Not that I suspect either is necessarily causing this but it's basic and fairly easy to do IF you can put the motor on a stand. Working over a transom is a pain.

If you could use an automotive timing light and 12 volt battery to check for spark when this occurs that could point you in the right direction.

All I've got for now.....

.....good luck

Thanks for the insight guys!

I have not tried full throttle with the hood off (engine is on transom and has a solar panel on a home-made "arch" over it so kind of hard to work on from the boat). I will have to try but with my understanding of it, if it was sucking up exhaust back into the intake and stalling, it should restart as soon as the hood is off and it has fresh air right? Last time it stalled I took the hood off to see if I could spot the problem and it still would not start back up after many minutes (and pulls )... so probably not am exhaust leak right?

Will also have to check temps. Cooling water seems to be flowing well and gets warm after a little while but I haven't serviced the pump and thermostat (bought used about 10 years ago). Do these engines have some type of thermal protection that cuts them off if they overheat? Also I find strange that it would run fine for many minutes at 50% throttle without overheating but then die after 10 sec at WOT. It would explain why it doesn't restart for a while after though!

Have not checked timing and valve clearances, will have to get a timing light and get the engine on a stand to do those, at the end of the season if possible if it doesn't seem like the cause of the problem...

Reading another forum post on a similar problem another guy was suggesting changing the plugs as they may be a bit too cold and when you go full throttle it has trouble igniting and the engine would flood? Does that make any sense to you ?

Thank you very much!

Well, sadly, these outboards don't have temperature sensors or any engine protection/ power derate strategy other than a vigilant helmsman. If it is getting hot enough to shut down during these episodes that would be bad. This sort of thing happening several times will damage the engine. Checking for that should be at the top of the priority list.

And as for the cold plug theory, I doubt it but anything is possible.

Your thoughts about how it should restart immediately after the hood is off I would think are accurate. If it was an exhaust leak stalling the engine, it should restart as soon as fresh air is available.

It's possible the CDI, pulsar coil, primary coil or ignition coil is failing. You can ohm check the coils but troubleshooting the CDI is something I haven't figured out how to do. There is a chart in the shop manual for checking resistances but using it and obtaining reliable results has not been attainable for me. I have no confidence in their test.

But the first step would be checking for spark when the issue is present.

Almost forgot....if it is flooding holding the throttle wide open while trying to retstart is the method typically used to clear it.

Good luck

Alright, I'll check the temperature first and foremost. I have a IR thermometer gun, what do you consider overheating and where should I measure exactly ?

Outboard motors are sensitive to the ambient water temperature they're operated in and that will have an effect on what temperature the engine attains. But the bottom line is that if cylinder head temp doesn't go above 180°f then she's not overheating.

Point your IR laser at two or more spots on the cylinder head as that's where the temperature will be the highest.

You also don't want to see it below 140°f after a bit of a run as that would indicate a stuck open stat. That would eventually lead to problems as well and you would want to replace it.