Honda BF8A won't run at lower RPM

[km1125]

It's a "new to me" Honda BF8A

Have had the motor for about a month and have put many hours on it. A few at full throttle but mostly something a bit less and also have idled around for hours and hours. I pulled the carb shortly after I got it and inspected and cleaned but it was pretty clean to begin with. It ran fine when I got it but I wanted to get an idea if fuel had been sitting in there in its prior life.

Yesterday we took the boat for a longer cruise. The lake was flat calm so we were able to run at 5800 for quite a while.. probably 20 or 30 minutes. Ran GREAT. When I slowed down at the end when I got to about 1/2 throttle it started to stumble. When I dropped down further it died. Never got even near idle on the throttle. When I tried to restart, it took a full choke to start, but I had to keep the RPMs up near 3000 to keep it running, but I also noticed that the green 'oil pressure' light was NOT lit like it normally should be (and was on the entire trip down). We let it sit for a while as we were at the sandbar but when we went to leave (3+hrs later) it was the same thing. I checked the oil and it was well over the min 200ml mark but nowhere near the full mark. I was able to get it started and make the run back at over 4000 RPM without issue but when we slowed again it died. The oil light did not come back on at all.

Back at the camp, I checked the oil pressure switch. It is open at rest and closes once the engine is running. I thought the light might have burned out, but later that evening I was able to get the motor to start and run at everything but below around 2000 RPM.

I am going through the carb again and chasing all the low speed circuits. It is the same keihen carb that's used in the Tohatsu/Merc/Nissan except a bit larger and jetted larger. Looking over the ignition, it also mirrors the early Tohatsu that had a separate exciter, pulsar, CDI module and coil, except the function on the oil light is inverted. None of the documentation I've seen so for indicates that the low oil will interrupt the CDI like the Tohatsu ones do though, but I'm not 100% convinced about that yet.

Anybody with experience on these?

 

[raczekp1]

start with carb leaning and spark plugs replace. next step is to hange twater pump impeler and thermostat.
and put new oil into engin. ryn motor and check oil preasure

 

[km1125]

I took the carb apart several times. It certainly looked clean, but after a few soakings (one overnight) I finally got out whatever was blocking it up. In the diagram below, I isolated it to where the red circle is. I could probe every other spot and verify they were clean and clear, except that spot. There is a plug pushed in there and I debated drilling it out but I would have had to find a replacement. The last soaking took care of whatever it was.

To be totally convince it was something in the carb, I took a carb from an old Tohatsu MSF5B and put it on there. I had to grind the fuel inlet casting part down a bit, but otherwise it worked good. The choke linkage wouln't fit because the Honda uses a larger diameter rod instead of a cable, but I was able to just manually put the choke in place. The engine ran fine with that smaller carb.

While I had it out of the water I did a bunch of other maintainance too... cleaned the thermostat, replaced the top shaft seal under the water pump and popped in a a new impeller.

Still have to troubleshoot the oil light problem though but at least the motor's running good again!!

 

[MattFL]

When cleaning out these 4T carburetors that have such tiny low speed circuits, you've got to blow cleaner followed by compressed air through all the little holes and jets to be sure they're clean. Soaking alone often won't do it. An ultrasonic cleaner can also help if you've got one. Glad you got it working again. Also since it's new to you, it would be a good idea to replace the fuel filter. If it sat at any point, some globs of goo may have formed on the clean side of the filter and could flow into the carburetor at any time, plugging things up. Depending on the year, you may also replace the gas lines. On my 50HP Honda (1999), last year I lost 1 cylinder while in the ocean and was able to drive home on 2 cylinders. Upon disassembling everything, a tiny bit of rubber from the gas line broke off and plugged the float pin in one of the carburetors.