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Honda BF75A 2004 RPM problem.

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beto_chamorro

New member
Hi everyone.

First of all, please excuse my writing. I’m not very good at writing in English, so I use Google Translate.

Let me tell you the long story of my engine. I bought it six years ago. I tested it and everything seemed fine, but for the past four years, I’ve been having problems with it.

The first symptom I noticed was that, with the throttle at 100%, the engine would start to choke after a few minutes and eventually stall. I thought it might be the carburetors, so I took them to a shop to have them cleaned. The mechanic asked me to bring just the carburetor assembly, saying he could clean and tune them without the engine. He mentioned he had a lot of experience tuning Honda engines.

When I reinstalled them, the engine ran more smoothly, but the same issue persisted. After a couple of minutes at full throttle, it would start to choke. It would reach about 3,500 RPM.

I thought the fuel pumps might be failing. I bought genuine Honda pumps and replaced them, but the engine's behavior didn't change.

From the day one, I noticed a lot of "dead travel" in the throttle—movement that didn't actually do anything. I thought it was normal. When I installed the carburetors, I saw that the throttle cable wasn't adjusted correctly and wasn't opening fully. I replaced the cables and adjusted them properly.

When I took the boat out, if I set the throttle to 100%, the engine would drop back to 3,200 RPM once it passed that mark and stay there. At least this time, it didn't stall.

I wondered if the fuel lines were the problem—perhaps they were old and letting air in. I replaced the entire fuel line leading to the pumps, including the engine connector. I also changed the fuel filters and synchronized the carburetors.

I went out on the water this Sunday, and this time, the engine held steady at 3,200 RPM. At one point, I opened the throttle fully; the engine revved up to 4,000 rpm and the speed increased, but only for about 10 seconds. Then the rpm dropped back to 3,200 and stayed there. I dropped anchor, and when we set off again, I was able to get the engine up to 4,000 rpm for another 10 seconds or so before it dropped back down.

While we were underway at full throttle and 3,200 rpm, I tried pumping the manual fuel primer bulb. It would firm up after 3 or 4 squeezes, but there was no change in the engine's performance, and the bulb would go soft again after a few seconds.

I’m completely stumped and have no idea what the problem might be. What tests can I run to start ruling things out? What do you think it could be?

Many thanks to everyone.
 
Hi Beto,

I Haven't worked on this model but I have some ideas about what might help.

Fuel supply hoses can break down internally and delaminate. Especially if left out in the sun. That could cause a piece of the hose to act as a valve and restrict flow during high flow demand when the throttle is opened. If at all possible, it is best to substitute another tank and hose when diagnosing a problem like this.

Check the vent valve on your fuel tank and make sure it's open all the way and is clean and unrestricted.

Check the fuel supply hose quick disconnect. Those will wear out over time. Make sure that the "push open" pin isn't worn down and the check ball doesn't have a flat spot.
Make sure that the latches lock in place and hold the disconnect firmly in place.

Additionally....

After consulting the Honda Marine Carburetion Manual on these BG model Keihin carburetors and checking the troubleshooting section for the symptoms you describe, it has this to say:

"Clean and/or replace Main Jets"
"Jet bore must be polished not just unrestricted"

The carb manual also shows the main jet to be located in a different location than that shown in the parts diagram. Having never worked on one of these I don't know which is correct but I might be able to provide you with a photo from the manual page if needed.

Interestingly, the larger 135 orifice "sea level" main jets (item 39 in link) are no longer available but the higher elevation jets (132 and 130 orifice) are still for sale.


If you need to open your carbs again to try and solve this, you should check the main jets to make sure they are the appropriate ones for your elevation.

I'm somewhat suspicious that your mechanic may have just tossed your carbs into an ultrasonic cleaner which would get them "somewhat clean" but probably wouldn't do a good enough job at all in my opinion.

Your writing in English was very good and understandable to me by the way. I am happy to discuss this with you anytime.

Good luck.
 
Thanks for the help with the language. I know English, but I struggle with writing. So I use Google Translate, but I have to write "badly" in Spanish, thinking about how it would be written in English to get a good translation.

As for the fuel line, everything has been replaced: all the hoses, pumps, and filters. I haven't replaced anything from the pumps to the carburetor, but I don't think that's the problem. That section of hoses works under pressure, not suction. If it were faulty, I'd see fuel leaking.

My question was whether it could be an electronic issue or if it sounds more like a carburetor problem. If I take the carburetors apart, I'll have to replace a lot of gaskets, and that's not cheap.

I preferred to be sure before taking them apart.
 
Yes, it could be any number of things on a used engine of this age. IE: valve operation or timing....ignition coils or spark plug wires...stator....pulser coil...CDI.
I don't have the service information, specific knowledge or the specifications to diagnose or adjust any of those

Your statement about fuel pump hoses isn't accurate. Just like a suction hose can break down internally and have a "flap" of lining act like a valve, the hoses under pressure can fail in the exact same way.

You've apparently already dismissed my suggestions about the quick disconnect and fuel tank vent as you made no mention of either so I'll not bother you further with any more other than to say it usually doesn't pay to get "tunnel vision" when trying to figure something like this out.

Good day.
 
I've ruled out the fuel line as the problem. The tank vent is working fine. When I disconnect the connector going to the engine and place the tube below the tank level, fuel flows without issue. And all the lines are fine because they're new, all the way to the pump. I doubt the problem is between the pump and the carburetor. I would expect a leak. And if there were a blockage, only allowing a little fuel to pass through, I should see some change when I manually prime the fuel pump, but I don't.
 
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