Honda BF25 Runs Poorly

[25inland]

I've been having trouble with the carburetors on the HONDA BF25 engine on my 13-footer.

In February 2024 I noticed the motor was hesitating when I was applying throttle. While returning to the harbor the HONDA BF25 was idling on two cylinders and shaking.

I use the boat almost every week, and the carburetors is not look very dirty. I took apart the carburetors and cleaned them with carburetor spray and a little scrubbing. I replaced the low-spped jets and the )-rings.

On March 8, 2024 ( a Friday) I was using the boat. The HONDA BF25 engine ran great for about one hour, but then, as I reduced throttle to idle for a while, the engine started to sputter and shake, and eventually stalled.

I was able to restart the HONDA BF25 engine and return to harbor, but the engine did not sound good and was down on power.

My next remedy was to mix a can of SEAFOAM into a one-gallon capacity fuel tank, and [to run the HONDA BF25 on this mixture for about an hour.

I was then back to "square one" with engine bogging at high-throttle settings and shaking and runnning roughly at idle throttle.

On March 9, 2024 (a Saturday) I soaked the carburetors in BERRYMAN'S CHEM DIP for an hour, and then scrubbed them as thoroughly as I could with a brass brush. I made sure all the jet [orifices] were clear with a torch-tip cleaner, and I reassembled [the carburetors]'

This brings me to day [March 10, 2024, a Sunday] with a HONDA BF25 four-stroke-cycle outboard engine that again ran great for several hours then barely ran at all.

I am beginning to wonder if something is contaminating the fuel, or if something in the fuel hoses [downstream of] the fuel filter is deteriorating.

I have very little experience with carburetors outside of a lawnmower.

Q1: what should I do next?

[25inland]

Later on March 10, 2024, I started the HONDA BF25 it in the backyard and it ran well for five minutes, then it reverted to running on two cylinders. I removed the spark plugs and inspected them, The spark plugs from cylinders one and three looked normal. The spark plug from cylinder two was back and had oily carbon deposited.

I measured the peak cylinder pressure in all three cylinders: cylinders one and three measured 190-PSI, and cylinder two measured about 175-PSI.

minutes or so, then went back to running on 2 cylinders. I pulled the plugs and cylinders 1 and 3 looked normal, but cylinder 2 was black with oily looking carbon. I did a compression test and 1 and 3 were about 190psi but cylinder 2 was around 175. HONDA specifications call for 215-PSI.

I now think it is time to re-power.

[dtmackey]

Q1: Are you getting spark on all three plugs when the engine appears to dros a cylinder?

While the compression is lower than what Honda states, it's not something that would cause [combustion to stop] in one cylinder.

I've seen situations like this where as the motor warms up, the heat causes something in the electronic ignition to fault and drops a cylinder. As a motor heats up it causes expansion of plastics and metals. I've tested engine electronics that are fine when cold and at fault when warm or hot. I'm not saying this is the [cause of the engine running poorly] I am trying to broaden the scope of what you've considered.

It takes three things to run:

1. fuel,
2. compression, and
3. spark.

Your status on investigation of the three

• Spark: no testing to confirm there is spark
• Fuel: if the engine runs on all cylinders and then doesn't, you cannot rule out fuel
• Compression: any [reading of a dry cylinder pressure measurement of] 175-, 190-, or 215-PSI will all [permit] the fuel to ignite, and thus I would rule out poor compression as the cause of the poor running.

The plug that is showing black oily carbon needs more focus. A picture of this spark plug will help readers make a better diagnosis.

If oil is entering the combustion chamber while the engine is running, there would be telltale BLUE SMOKE in the exhaust.

On that basis, [my diagnosis of the cause of the problem is] leaning toward a problem with either FUEL or SPARK.

[jimh]

The general situation with any electronic assembly is that the performance will vary with temperature, and it is generally very well known that as the temperature rises in an electronic assembly the chance of a malfunction increases. In my own experience with outboard engines which displayed an intermittent loss of combustion in one cylinder, the cause was due to loss of spark in that cylinder due to an electronic or electrical component that began to malfunction as the engine temperature rose and heat was being transferred into that electronic or electrical component from the engine.

A very simple way to establish that there is SPARK in a cylinder is with use of an IN-LINE SPARK TESTER. A very excellent product is made by LISLE and is sold in many stores (such as auto parts stores) that offer engine testing equipment is the LISLE 20610 SPARK TESTER. See FIgure 1 below for an illustration of the LISLE 20610 SPARK TESTER.


Fig. 1. The LISLE Spark In-line Tester, part number 20610

This very useful too is still available and--quite amazingly--is still priced under $20. See the LISLE website for more information and instructions on how to use the test at the hyperlinked URL below.

https://www.lislecorp.com/specialty-tools/inline-spark-tester

Q1: what should I do next?

I recommend you do the following, in this order:

1. buy a LISLE in-line spark tester
2. remove the engine cowling
3. insert the LISLE in-line spark tester into the spark plug lead at the cylinder which you suspect is the one that is losing combustion
4. with the engine cold, and not at high noon but rather in a part of the day when the sunlight is not so strong so observation of the spark in the spark tester will be made more easily, start the engine
5. verify that you can see spark jumping across the gap in the LISLE spark tester
6. operate the engine in the same manner that you have done previously that seems to bring on the poor running
7. when the engine changes to poor running, observe the LISLE spark tester to see if the chosen cylinder still has spark
8. if the chosen cylinder still has spark, move the tester to another cylinder, and check for spark
10. repeat on all cylinders until you find one that has no spark
11. return to the forum and report the outcome of the test

[jimh]

...It takes three things to run:

1. fuel,
2. compression, and
3. spark.

The general category for the "compression" is more often noted as "oxygen."

In order for combustion to occur, the mixture of air-to-gasoline fuel needs to be in a proper ratio, which is established by the stoichiometry of the chemical reaction that is the burning or oxidation of the gasoline. Whether or not there is an elevated pressure when this is to occur is not absolutely necessary. For example, my gas cooking range will ignite its natural gas fuel when at atmospheric pressure, that is, without an elevated pressure.

A typical gasoline internal combustion engine will have compression occurring due to the upward movement of the piston in the cylinder, but typically the amount of compression is more likely a determinant on the amount of force that will be exerted on the piston when combustion occurs than whether or not any combustion will occur. When the fuel-air mixture in the correct ratio is compressed by the upward movement, there is an increase in temperature, which helps to encourage combustion and cause the liquid fuel to vaporize. The spark is timed so it occurs very near the peak of upward movement of the cylinder, when the volume contained in the combustion chamber is the smallest. The chemical reaction that occurs with combustion results in conversion of the the air and fuel into a much greater volume of carbon dioxide and other gases, which is what forces the piston downward. So compression is needed to extract power from the combustion process, but it is not absolutely necessary for combustion to be created--although I am sure it helps.

PEAK CYLINDER PRESSURE READINGS

I measured the peak cylinder pressure in all three cylinders: cylinders one and three measured 190-PSI, and cylinder two measured about 175-PSI.

As for measurement of the peak pressure measured in a cylinder by removing the spark plug and substituting a pressure gauge, the notion that an engine must exactly match the factory specified pressure when the engine was new in order for combustion to occur is not really a good guide.

The measured value of peak pressure could vary depending on the gauge being used and whether or not the cylinder walls were wet with oil or how much wear as occurred on the piston rings and the cylinder walls.

To infer that an engine must be abandoned and replaced because one cylinder of three shows a small deviation from a factory specification is not justified.

[25inland]

Thanks for the suggestions.

I bought the LISLE in-line spark tester. The number-2 cylinder seemed to have strong and [continual] spark.

I also [substituted] an ignition coil and wire (from another engine I have access to). The HONDA BF25 still ran exactly the same as before.

The spark plug from cylinder-2 is shown below in Figure 1.

Fig. 2. The spark plug from the cylinder-2.

sparkPlug_.jpg (38.53 KiB) Viewed 928 times

The spark plug smelled of gasoline when pulled, and so did the engine exhaust while it was running.

Q2: could something be causing fuel to dump into cylinder-2?

The only thing I recall doing differently to the cylinder-2 carburetor was raising the float height by 1-mm to the HONDA specifications of 14-mm. This is done with the carburetor upside down, and to my understanding this would cause the float to push the needle valve closed with a lower fuel level in the bowl.

[jimh]

I bought the LISLE in-line spark tester. The number-2 cylinder seemed to have strong and [continual] spark...The HONDA BF25 still ran exactly the same as before.

I am inferring that what you are reporting was a test situation where after the engine temperature rose to the normal hot engine range and the engine exhibited poor running characteristics, you still had spark indicated on cylinder-2. If that is the case, then lack of spark in cylinder-2 does not seem to be the cause of the poor running. Having a gasoline smell seems to indicate there was un-combusted gasoline in the cylinder. This probably indicates that the mixture of fuel and air introduced into the cylinder was not in the proper ratio.

Q2: could something be causing fuel to dump into cylinder-2?

Your question seems to presume the cause of no combustion in cylinder-2 was due to the fuel-air mixture being too rich on fuel. It could also be due to the mixture being too-lean on fuel.

As for exactly what mechanical defect could cause either of these could occur I will have to leave the answer to this to others who are more familiar with the intricacies of carburetors on a HONDA BF25.

I will speculate a bit on the possible cause of the too-lean mixture possibility: there is just not enough fuel being delivered, which could be due to one of the fuel orifices being obstructed or incorrectly set. Usually these orifices are threaded and are adjusted by rotating them in or out a few turns. Usually the Service Manual will give advice on how to adjust them.

I will also speculate a bit on a possible cause of the the too-rich mixture possibility: generally in a gasoline engine with carburetors there will be an enrichment circuit that tries to increase the amount of fuel provided in order to encourage starting the engine when cold, often referred to as a "choke" because it can be implemented by reducing the amount of air by keeping the throttle plate closed. But in some designs, the enrichment is created by introducing more fuel than normal. Exactly which method is used on the HONDA BF25 is unknown to me. Nor do I know how the enrichment is precisely accomplished.

You may be able to learn more about the fuel enrichment method from the Service Manual for the BF25 engine.

ASIDE: This may not be true on your HONDA BF25 engine, but on an older OMC V6 engine that I owned, there was an electrically-operated valve that supplied added fuel for starting the engine. There was also a mechanical valve in this same circuit. If the mechanical valve was inadvertently opened the result would be the engine would have much too much fuel and would not start. Something akin to this might be going on in your HONDA BF25 engine.

[25inland]

I disassembled the carburetor from cylinder-2. The cause of [the engine running poorly] was immediately apparent.

Fig. 3 The bowl from the carburetor from cylinder-2 with odd yellow debris in the botton; see inset at upper right for close-up view of the yellow debris.

carburetorBowlWithInsert.jpg (48.58 KiB) Viewed 880 times

Q3: where did the yellow debris seen in Figure 3 come from?

[jimh]

Q3: where did the yellow debris seen in Figure 3 come from?

My first thought was to ask you if while you were working on the carburetor were you eating potato chips?

More seriously, speculating on the source of the yellow debris (discovered in the carburetor bowl from cylinder-2 of the HONDA BF25 engine that seems to run poorly after it has warmed up and is believed to lose combustion in cylinder-2), my first thought is that the debris could not have come from the fuel being used. There is no basis to support the idea that debris in the fuel was always and repeatably settling only in the bowl for the carburetor on cylinder-2. If there were debris in the fuel, it would be just as likely to be found in the carburetors for all cylinders. And here I am assuming that you HAVE already inspected the bowls of the other two carburetors and did not find any odd yellow debris in them. If not, then that action should be your next move.

My second thought is the debris must be something insoluble in gasoline. Gasoline is a very good solvent, so whatever the yellow debris is from, it must be insoluble in gasoline.

On the basis of those two assumptions, a reasonable theory of the origin source for an insoluble yellow particulate that only affects the carburetor for cylinder-2 would be a fuel hose that feeds only the carburetor for cylinder-2.

A troubling aspect of the new theory (that the yellow debris seen in the bowl for the carburetor from cylinder-2 is the cause of the habitual problems with the engine running poorly) is the notion that you have previously disassembled the carburetors for cleaning at least twice (as I recall from your narrative) and apparently did not notice the yellow debris. I would expect that if the yellow debris were the cause of the poor running, the yellow debris would have been present every time you disassembled the carburetors.

Q4: is there a possibility that the yellow debris has always been present and that the two earlier times you disassembled the carburetor you did not notice the yellow debris in the bowl?

[dtmackey]

Thanks for the update on the spark and using the same tester as Jim mentioned. In my 40 years of engine work, I've never used a tester and rely on a sparkplug witht the metal body touching the engine block.

Now that we know the cylinder is getting spark—and I'm assuming you did this [both] cold and warm after the misfiring started.

Q5: are the carburetors able to be swapped?

I know the linkage may need to be disconnected, but this would let you know if the [loss of combustion] moves to another cylinder when the "suspect" carburetor has been moved to that cylinder.

Q6: in Figure 3 of the bowl with the yellow stuff, do the other bowls exhibit the same yellow stuff?

Q7: are you using ethanol-blended fuel?

Sorry to pepper you with questions, but I've worked on plenty of small horsepower enngines where E10 ethanol fuel has caused poor running.

Ethanol in gasoline fuel pulls in moisture and this moisture in the fuel combined with the corrosive properties of ethanol in the fuel reacts with the pot-metal that is used in carburetor castings. Usually this results in a white corrosion and this clogs carburetor passages downstream of the bowl. The bigger the engine horsepower the less chance of this process occurring as the passages are larger and allow passage of this material, but the smaller the motor, the bigger the chance for this to occur.

The chance of this problem occurring also varies on engine manufacturer.

I've found two-stroke-power-cycle engines are less prone to [fuel contamination from water in the fuel coming out of solution and creating a harmful and corrosive material] because the oil in the gasoline minimizes corrosion. In four-stroke-power-cycle engines this process does seem to occur. I've seen several four-stroke-power-cycle engines in the 4 to 15-HP range where this happens in a matter of weeks.

Using fresh gasoline decreases the chances of [ethanol pulling water out of solution in the gasoline and then forming a corrosive liquid], but to monitor [for this process occurring] is a headache.

Just more to think about.

D-

[25inland]

in Figure 3 of the bowl with the yellow stuff, do the other bowls exhibit the same yellow stuff?

All the other carburetor bowls did indeed have the same yellow debris inside them, though maybe not quite as much as the carburetor on cylinder 2.

Q5: are the carburetors able to be swapped?

The carburetors from cylinders 1 and 2 can be swapped and have the same part number but the carburetor on cylinder number 3 is a bit different and I'm unsure if it can be swapped. than the other two. I did reassemble the carburetors after clearing out the debris and it ran wonderfully in the test tank in the backyard for about 10 minutes.

I checked the bowls after running it again and found some similar debris but in very small amounts.

Q7: are you using ethanol-blended fuel?

Unfortunately in my area I can only buy E10 fuel due to emissions regulations. (Thank you CARB.) The local fuel dock sells fuel with a fuel stabilizer additive already blended in the fuel but it seems like it might not be enough. The boat is used fairly often so the fuel isn't very old.

A troubling aspect of the new theory (that the yellow debris seen in the bowl for the carburetor from cylinder-2 is the cause of the habitual problems with the engine running poorly) is the notion that you have previously disassembled the carburetors for cleaning at least twice (as I recall from your narrative) and apparently did not notice the yellow debris.

It is true this is the first time I've noticed the debris but I suppose it's possible I could have missed it the first time, or perhaps the pieces weren't quite as large and noticeable but still able to clog the tiny jets.

Other forum posts mentioned fuel can varnish in the fuel lines and flake off when the lines are pulled and flexed when removing and reinstalling the carburetors from the intake manifold. I feel. that given the age of the 2002 HONDAY BF25a2, the fuel lines could be original. Because I know the previous owner (a friend) and that he used the boat infrequently and didn't use stabilized fuel, this could be a plausible theory.

In any case at the very least it might be good preventative maintenance to replace the fuel lines.

Q8: should I replace the fuel lines?

[jimh]

From the mention of CARB, I infer you are in California. You should be able to buy pure gasoline as aviation fuel. You might check with a local small airport fixed base operator to see if you could buy a few gallons of aviation fuel as a test.

Your new observation that you are now able to observe the yellow debris in ALL three carburetors allows for the fuel, the fuel tank, and the fuel hoses from the fuel tank to the engine's fuel fitting to be suspects as the origin of the debris.

One source of fuel contamination that is well known is from fuel hoses that have a metallic gray appearance. These hoses are FAMOUS for being degraded by use of ethanol-gasoline blended fuel. They have a liner material on the ID of the hose that is affected by the ethanol and breaks apart. If you have ANY fuel lines that are gray and have a metallic finish, get rid of them. Replace those gray hoses with black rubber fuel hoses that specifically state they are tolerant of ethanol-gasoline fuel. By the way, the prices of those rubber hoses is quite crazy. I think I recent paid about $6 a foot for some.

If possible, you should immediately test the engine by running it from a different fuel tank, a tank known to have no debris in it, and with proper rubber fuel lines that are not likely to be degrading from use with ethanol fuel. And running on pure gasoline could also be included in the test.

[jimh]

ASIDE

Thanks for the update on the spark and using the same tester as Jim mentioned. In my 40 years of engine work, I've never used a tester and rely on a spark plug with the metal body touching the engine block.

The very significant advantage of investing in the under-$20 LISLE spark tester is that you can very easily observe the spark while the engine is running and see if the spark suddenly goes away when the poor running occurs. The tester is also very useful in avoiding getting a rather big jolt from the spark voltage if the spark plug is not held continuously in contact with the engine chassis. In the total cost of boating, the $20 is well spent.

[25inland]

The fuel tank and hose from the fuel tank to the outboard are only about a year old and there is a filter in between them and the carburetors so I feel like that would be a less likely source of the debris. The hoses distributing the fuel to the fuel pump and the carburetor are a matte gray and look different than the OEM hose that Honda currently produces which are of the solid black type that you mentioned Jim.

Some other posts described "yellow gelatinous flakes" coming out of their old gray fuel lines which closely matches what I've been finding in my carburetor bowls.

I have ordered some new Honda fuel line and will be replacing everything under the cowling. I may also install a new primer bulb.