HONDA BF20 HP runs rough and quits after warm-up... but I think I found the issue

The Honda 20hp seems to run perfect until it gets hot. After it warms up, if I try to give it gas, it will sputter then quit. Then restart it seems to idle fine, but eventually shuts down.. again and again. I tried taking off the carb and doing a thorough clean (twice), but the symptoms were exactly the same. I tried new gas... no difference. Finally, I took off a cover on the back of the engine just above the propellor and revealed small bits of black rubber. I figured these little bits were pieced of the water pump impeller. I removed the propeller and gear case and got to the water pump... turns out the impeller is completely destroyed. So no cool water was being delivered to the engine and it seems to have been shutting itself off. Smart engine. I hope I didn't do too much damage by trying to run it.
My concern is this... what about all the other little bits that are probably somewhere in the engine? Is there a way to back-flush the system to try and remove all of the bits. What caused this impeller to disintegrate? Not what I expect from a high end Honda.
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How old WAS the impeller? They usually don't last more than two or three years....no matter who manufactured the outboard. And, if the outboard was EVER started with the motor tilted up so that the impeller ran dry...even for just a few SECONDS...that kills them QUICK!

While you have the pump apart and the gearcase removed, you can backflush the engine by pulling the thermostat and forcing water into the block and head passages located behind the stat and in the stat housing cavity. Any stray pieces of old impeller should come out the extension case where the water tube from the pump hooks in.

You probably didn't do much damage, because Honda, through the ignition control module, forces engine depower when it overheats. The fact that she still starts and idles ok indicates a STILL healthy outboard. The Honda BF 20 is, POUND FOR POUND, the MOST reliable and rugged outboard sold. IF maintained and operated PROPERLY.

Good luck.

Thank you for the help. I have read a lot of other forums about folks having the same symtoms but not a lot of successful fixes. Most suggestions involve the carborator, but no one ever seems to follow up with the solution to the problem. Perhaps the broken impeller a common maintainance issue that is overlooked. The impeller is probably the original from 2011 so that explains why it's gone. Any idea why it's not made from a more resilient matierial? It seems just like flimsy rubber. Thank you for the tip on flushing from the thermostat. I took the thermostat cover off yesterday and didn't see much debris but it's got to be in there somewhere.

Sometimes the debris will work it's way down...UNDER the pump impeller. So keep that in mind as you look things over. Also try and backflush the tell tale indicator passage while you're at it. Things lodged on the intake side of the pump can be removed by taking off the intake screens on the gear case and fishing it out. To clear the tell tale passage, take off the flexible tube from the fitting in the block and remove the fitting to flush anything out of there. Otherwise, you may be trying to clear it repeatedly as you see the pee stream diminish. Also, the routing of that rubber tube is CRITICAL to a properly operating tell tale stream. If installed incorrectly, it can be pinched closed between the oil case "ledge" and the side cover. That tube is also a regular maintenance item too. It tends to become hard over time and will trap small little twigs and such much more readily. Replace it every two seasons.

The impeller can actually be fairly reliable for about 3 years but I don't know of ANY manufacturer that doesn't recommend replacing them once every season. If they are run dry...even for an instant...all bets are off.

The reason they are made like they are is the environment that they have to survive in. There is MUCH debris encountered and almost NO filtration prior to the water entering the pump. Ironically, if the pump were made with robust titanium blades, instead of flexible rubber arms, it would probably be beaten and smashed into small particles in a matter of hours on a twig littered lake or stream.

If you can come up with a "maintenance free" outboard water pump, you could become a VERY rich individual almost instantly.

Good luck.

thanks for the advice. It's sort of like the engine has kidney stones and needs to pass them. poor thing. I think a secondary filter screen somewhere above the pump might be a good idea... just to prevent bits of impeller from going into the engine, but then theres a risk of the screen coming dislodged and getting stuck somewhere. Anyways. wouldn't be able to get it fixed right without the guidance on here. thank you.

Pull the thermostat and back flush with a garden hose from there. You may also find pieces of rubber in the thermostat. Also make sure that the thermostat is not stuck open from the heat.

Also look for them in the water intakes....as Jimmy says....the pieces fall under the pump base and sometmes all the way down.

Most often impellers fail by the owner or previous owner cranking and starting the engine without water going to it.

Mike

I should have said: If you can come up with a maintenance free water pump that doesn't cost more than the outboard, you could be rich. The rubber impeller is just as much about cost as it is resilience. But, it's what we all have to work with.
Good luck and let us know what happens.

I installed the new impeller. Before installing the side cover, I tested the motor and a nice steady stream of water was shooting out the tell tale indicator tube. After putting the cover back on, the stream still seemed to be constant, but a little diminished. I think the outlet is constricting the flow a bit because it's much narrower than the tube. When I gas the engine, it seems to have a nice pee stream. The engine didn't shut down on the first test so I think it's fixed! thanks for the help.

Yeah Buddy!
Sounds good. And, you did this all on your own....You troubleshot it and came to a rational decision and fixed it.

Yes, the tell tale "nozzle" does restrict the flow a bit and I have, in the past, opened that up because it kept getting these little pieces of bottom "grass" caught up. All the rest of the passages would "pass the grass" but that nozzle would choke on it and cause me headaches....29 boats....all being flagged for bad pumps! A very small drill fixes that.

And don't forget that the rubber tube gets "hardened" and should be periodically replaced.

Way to go!