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Honda 1999 40 hp BF40 on Pontoon Boat

FredsBoat

New member
I have a 20' pontoon boat with a Honda bf40 hp (1999). I use to get about 5300 rpm's out of it. Now I only can get a max of 3800 rpm's. Any suggestions?:(
 
Re: Honda BF40 on Pontoon Boat

A number of questions...

1. Was this change gradual or all of a sudden?

2. Have you changed or damaged your prop?

3. Are you carrying more weight or people in the boat?

4. Does your motor run smoothly at idle? At higher speeds?

5. Have you tapped on your tachometer to be sure it is not stuck?

6. Have you operated the choke to see if the engine picks up in speed?

7. Are you using fresh fuel?

8. Have you checked the fuel lines to and inside the engine for leakage or pinching?

9. Have you changed your fuel filter?

10. Have you changed your spark plugs? Only use NGK DR7EA.

As you can see, it can be many things. For example, if you leave your pontoon in the water all the time, there can be buildup on the toons that can slow the boat down, which also affects the rpm.

You could also have water in your toons that is loading it down.

The motor may not be functioning correctly.

If you changed your prop, you may have put a higher pitch on it, thus lower rpm.

There are many other things that can cause the problem. You have to check the basics first.
 
Re: Honda BF40 on Pontoon Boat

1. It's my dad's boat and he thinks it changed about 2 years ago.
2. The prop is not damaged. it is a 11.6 x 11
3. Just one or two people...well within weight limits
4. Motor seems to run smooth both at idle and higher speeds.
5. Tac is working fine
6. Not while running....I will try it and let you know.
7. Yes.....Fresh fuel
8. Fuel lines are clean and not pinched.
9. Fule filter changed recently.
10. Spark plugs are good...recently changed.
The boat is stored on a trailer when it is not running. I also wash the toons down each time it is taken out of the water.
You mention that the prop may cause lower rpm. As i mentioned above, the motor currently has a 11.6x11....My dad is not sure if that is the same size prop that came with it or not. Should I try one that has a lower pitch? If so, what size should I try?
Thanks,
 
Re: Honda BF40 on Pontoon Boat

The pitch of the prop is probably a little big. Should probably be more like a 9 or 10 pitch. That being said, a 9 pitch will not make up 2000 rpm difference.

Is it possible that your tach is not accurate? Could it just not be able to go past 3800 rpm? It would not be the first time I saw that.

I know you said that the engine runs smooth but...does it idle (in neutral) about 950 plus or minus 50 rpm?

I would be good to do some basics on the engiine.

If you have the capability, it would be good to do a compression test on all of the cylinders.

It would also be good to do a cylinder drop test.... With the motor running in neutral, momentarily pull the spark plug wires off of each cylinder one at a time. Use an insulated pliers. As you pull off each spark plug wire, the engine speed should decrease but not stop. You should only have to pull it off just a little for about a second to get a reaction.

If the engine speed does not decrease, then there is an issue with that cylinder...and you have an engine problem.

We just have to start at basics. Just like when you go to the doctor, he/she takes your blood pressure and listens to your lungs and heart.
 
Re: Honda BF40 on Pontoon Boat

Update...
We went to a 9 pitch from an 11 pitch and the rpms dropped 400 rpm's. We also did a cylinder drop test. the rpm's dropped about ~ 200 rpm's when each of the top two plug wires were pulled along with noticable roughness. When the third (bottom) was pulled, there was little drop in rpm's ~50 rpm's.

Thoughts?
 
Re: Honda BF40 on Pontoon Boat

The loss in rpms going to a smaller pitch does not make sense to me. They should have gone up about 400 rpms or so. Were the diameters of the props the same size?

As for the cylinder drop test. You may have an issue with the #3 cylinder.
It could be as simple as the idle mixture screw for that carb is not set correctly or it could be something else.

Try this....
First, you have to find the vertical plastic choke linkage (about 8 or 9 inches long and about 1/4 inches wide) that connects all the carburators together. You can stand next to the engine and manually operate the choke and see it move up and down. It is black plastic.
With a needle nose pliers, carefully pop it out of each carburator. It should not take much effort.
Now...when you run the engine pull the choke knob out momentarily on the front of the engine and see if the engine speed picks up. The knob will only operate the choke on the bottom carburator.

If the speed picks up, then you probably have a internal blockage in one or more of the passages in the bottom carb. That carb is the most likely to have a problem, since the fuel settles there even if you run the engine out of fuel. Over time, it starts plugging up the main jet and others.

That means, it is time to clean the carbs. I recommend cleaning them all or having someone do it for you.

But let's see what happens in your test before we get too far into the goo.
 
Re: Honda BF40 on Pontoon Boat

Hi guys,
I've been lurking here watching this thread because it's interesting. One thing that struck me is that when you divide the 5300 rpm he used to have by the number of cylinders, you get around 1750 rpm and the difference between 5300 and 1750 is 3550 or, very close to what he has left now. It sure appears that you can directly read the rpm drop as a missing cylinder. A good thing to know when trouble shooting a 3 cylinder.

The lost rpm after the prop change is a puzzler though. Questions I have are;
Is the new prop the same diameter as the old one?
Does the new prop have the same number of blades as the old one?
Is the new prop a different brand/style from the old one?
Are you making the test run in the same water and wind conditions as before?
Is the boat loaded the same as before?

At any rate, the prop issue shouldn't be pursued until you solve your engine power loss. If Mike is right (and he usually is) you'll get that cylinder back along with the missing rpm with a good carb cleaning.

If the power loss is from a spark issue though, you will not want to run that engine anymore than for troubleshooting purposes if the carb is feeding the cylinder fuel and it isn't being ignited. The raw, unburned fuel washes the oil off of the cylinder walls and will cause rapid ring, cylinder and piston wear.

It being a 99, it's certainly not too early for a compression check and valve adjustment.

It is amazing how well these things will run with a missing cylinder!
 
Re: Honda BF40 on Pontoon Boat

Here is the answer to some of your questions...

New prop size is 11.6 x 9 .....old prop size 11.6 x 11
Both props have 3 blades
same brand and style
test conditions were the same
boat was loaded the same.

I'll do the things suggested early next week and keep you updated.

Thanks....Happy Easter.
 
Re: Honda BF40 on Pontoon Boat

Here's the update....with the engine running we pulled the choak linkage off. Then we momentarly pulled the choak out and there was NO change in RPMs. If we left the choak out more than about 4 or 5 sec. the engine started to choak out. Suggestions....:confused:
 
Re: Honda BF40 on Pontoon Boat

You could try running a double dose of Sea Foam or Stabil Marine but if there is a major blockage, it probably will not help.

Runability problems generally are either a compression, spark/timing, or fuel problem.

In one of my previous posts, I suggested a compression test. So we still do not know if compression is where it should be. Can you do a compression test?

Not having a compression test is like a doctor trying to figure out why you are having chest pains without at least taking your blood pressure.

Before we go to check spark...might as well finish up testing each carb for lack of fuel. It probably will not tell us anythiing, but we might as well do it to complete the fuel test. As you could probably see when you disconnected the choke linkage, you can now operate the choke on each carbuator individually. Again, just a quick flip or two of each choke lever and see if there is pickup. If no pickup, do they all react the same?

In case the yields us nothing...

Go back to pulling the spark plugs wires one at a time. This time, keep the connector close to the spark plug and listen for the ticking of the spark jumping onto the spark plug. At idle, the ticking should be fairly slow. Listen first for ticking at each spark plug. Pay close attention to the speed of the ticking compared to the other cylinders. They should all be the same. If one (possibly the bottom cylinder) is faster (like another 1/3 faster) than the others, then we definitely have a spark issue. If there is no ticking at any cylinder, that is not good. If they are all the same, the spark is probably ok, although this is not an absolute since we are not using a spark checker.

If one cylinder had no ticking, swap that spark plug with another one and see if the problem follows the spark plug.

One repeat of a question in an above post...did you change the spark plugs before or after you started having the problem? If it was before or you can not remember, change them again. NGK DR7EA

One other major item can cause it to run like this....timing. If the timing belt has jumped just one notch, it can run like it is a carburator issue.

Have you ever purchased a repair manual yet? The manual explains how to check the timing. It is quite difficult to explain how to find the marks without pictures.
 
Re: Honda BF40 on Pontoon Boat

We changed the spark plugs last week.
I will take it and have a compression test done.
I do have a manual.
I will check the spark later today.

Thanks,....
 
Re: Honda BF40 on Pontoon Boat

Fredsboat,

Do not forget to check the timing. Check out p 3-31 in your manual for the timing marks. If you have a cover on your flywheel, you will have to remove it to see the flywheel marks.

If you take out your spark plugs, it is easier to turn your flywheel by hand. Be sure to turn it counterclockwise...there is an arrow on the flywheel. This direction is opposite of most motors...if you go the wrong way, you could mess up your impeller.

There are a lot of pictures but you only need two. The T mark on the cam pulley and the T mark on the flywheel.
 
Re: Honda BF40 on Pontoon Boat

It's not a spark problem. I have worked on Honda's for years and they very very rarely have a spark problem. I am 99.9% sure that you need to clean out the bottom carb. To clean the carb, you have to remove the intake manifold and remove all three carbs at the same time. Disassemble the carb and clean out all the jets with carb cleaner and compressed air. If you need any advice on how to properly clean out a Honda carburator, email me at [email protected].
 
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