BF8 Engine speed going up and down

I just changed the fuel filter and the spark plug in addition to having the carburetor cleaned by a repair shop. The engine starts well and is running fine at idle and at low and medium speed when in neutral. At the dock, I put the shift lever in the forward position and let it run at low and medium speed. All of a sudden, without even touching the tiller, the engine speed starts running high and then goes down after 5-10 seconds. It does not stall. The propeller is clean and turns freely. Could it be something with the fuel pump, e.g. the membrane?
Thank you.

I suppose it could be the pump causing the engine to go lean but if it were I think the engine would stall, not recover.

I suggest that you check other basics first.

If you have an "open" fuel tank, make sure the fuel line is properly connected and latched. Check the fuel tank vent and make sure it's open and clear. If the hose is old and showing age or cracks you may want to replace it since it might be sucking in air.

If you have a new sealed fuel tank, try pressing the bypass on the Fuel Demand Valve (FDV) to see if there might be a blockage causing the engine to lean out.

When it starts act up, squeeze the primer bulb and see if that makes a difference.

Try tapping the carburetor float bowl to see if your float could be sticking.

QUESTIONS: 1. What year is the outboard?

2. Is this "symptom" why you had the carburetor cleaned and replaced the spark plugs?

3. What spark plugs did you use?

4. Is this a recent purchase and came with this problem or have you had it for a while and it ran ok until this started?

Good luck

Thanks for your reply.
I will check the fuel line as you suggest. It could be dry and cracked.
The outboard is a 2006. I had it since 2014 when it came with the boat. It was running fine until this Spring.
I had the carburetor cleaned to solve a first problem. I flooded the engine trying to start it. Once the spark plug and fuel filter were replaced, it started easily. I must admit the the engine had not been serviced for a long time.
The spark plug is a NGK from the Honda dealership.
I will check the diaphragm in the fuel pump. The membrane might be dry and cracked.

Hi Dan,
Honda doesn't sell service parts for the fuel pump so you would likely need a new one if you find a problem with the diaphragm.

https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/8hp/bf8d6-sha-2006/fuel-pump

When the diaphragm cracks it can spill fuel into the oil via the valve cover and head. Check your oil carefully for fuel contamination and immediately change the oil and filter if you think it has been diluted.

The carburetor doesn't use a conventional butterfly type choke but, rather an SE (start enrichment) valve that the choke cable manipulates up and down. See items 12 and 20 in the link below.

https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/8hp/bf8d6-sha-2006/carburetor-manual

The SE valve has some weaknesses that can and do cause problems. The valve screws into the carb body with weak, plastic threads. I have seen these threads destroyed over time by engine vibration. I have also seen the threads damaged by careless installation (crossthreaded) and over tightening. When the threads get damaged, the valve assembly will "float" inside the hole and usually cause hard cold starts.

The valve also has a curved, rubberized boot that the choke cable is inserted in. This boot flexes and will age, crack and split open over time. When that happens, hard cold starts are usually the result but it could also cause other running issues. I'm not saying that the SE valve is causing your current problem but it is something to be aware of.

Keep us updated on what you find.

Good luck.

Thank you. I will check those asap.

I neglected to say that, in addition to checking the SE valve boot for cracking or splitting, you should pull and push the choke cable while observing how the valve responds. Any up and down movement or even slight rotation would indicate that the valve is loose and possibly has damaged threads.

Also, if the boot seems to "swell up" when you push the cable inwards that could indicate that the cable end has become unattached from the valve and is buckling inside the boot.

If and when you ever decide that the SE valve needs to be replaced I highly recommend that you replace both the valve and the cable at the same time. Because, although it looks as if you could reuse the cable with a new valve and save a few $$$, I had no luck trying that when I was maintaining these outboards on a daily basis.

Here is a link to a short clip shopping how the engine reeves up by itself when in forward gear . Hope it works.
https://vimeo.com/579860244

If that wasn't you coughing or sneezing at about 14 seconds into the video then that sounds like it was backfiring, or trying to, out of the intake. A sort of classic indication of a lean condition or valve timing being off or both.

Are you in the habit of running the outboard fuel system dry when parking the boat? If so, the way the engine reacts might make this familiar to you.

That video reminds me of what many engines will do as the fuel in the carb goes low and the engine starves out and then quits. Only in this case, it picks back up like it's getting fuel again.

Have you checked the the things we discussed earlier? Tried tapping the float bowl while this is occurring?

I'll be honest here and tell you that I have very little trust of most shop's ability to get these little finicky carbs set right. There are many little things that NEED to be done. I know, I've done many of these and struggled with that myself in the beginning. A very WISE Honda tech talked me into buying the HONDA MARINE CARBURETION MANUAL and I sure wish I had had one from the start. Most shops don't have that crucial information and that turns cleaning the little critters into a game of trial and error or a "wash, rinse, repeat" scenario.

Having said that, the fact that it starts easy and seems to idle ok would indicate that they got it at least mostly right and I still think that it's either pulling in air or losing prime momentarily for some reason. But if you can't find a fuel FEED problem then it might very well be a carb issue that I've just never encountered before.

Good luck.

Going back over what's been said it strikes me that you don't mention how the outboard runs under power. Has this problem kept you from taking it out on a thorough test drive? Is that why you just talk about running it in gear at the dock?

Just mulling this problem over and gathering info.

Good luck.

It was me coughing…
I made the video a few days ago. I will be going at the boat tomorrow and check what you have suggested.
i only ran the engine at the dock. Did not take any chance leaving…

Ok,
Well, i had another thought. A bit of a long shot but you could easily check it out.

When I worked on a fleet of 8D rentals I was struck by how easily they would start and how smoothly they would run on just one cylinder. That is, until you needed some power of course. But it's amazing how they can fool you.

I wonder if she's only running on one hole and then the other kicks in and she revs up? It's possible but that would mean that the shop missed that and set the idle way up when they reinstalled the carb. Or, did you install the carb?

At any rate, if you take a thick glove or spark plug wire pliers with you along with some dielectric (tune up) grease when you visit the dock, you can do a power contribution test to find out.

Just remove the spark plug caps and smear a little grease inside the boot edges. This makes pulling them off the spark plug and putting them back easy.

Start the outboard and pull one wire off noting how the engine responds. Then put it back on and repeat with the other cylinder.

The cylinder that causes the least amount of change when the plug wire is pulled off is the one contributing the least. The weak cylinder.

If pulling a plug wire causes no change, then it's likely that cylinder isn't firing at all.

It's a simple test that yields valuable information sometimes.

P.S. smearing dielectric grease inside spark plug boots is a common and necessary step when changing plugs or wires. It's a good thing. Just use it sparingly.

Good luck

Sure sounds like its running out if fuel then it gets fuel again. While running normal pull fuel line off and let it run until its out of gas. See if its the same symptom. If so its either float, fuel pump, lines or connectors.

Also check vent on tank or run with cap loose see if it runs normal.

Also try pumping bulb while running, it should stay firm. If pumping fixes it then fuel pump is bad.

You could also try holding tank higher then motor.

The SE valve is weak and the threads are almost non existant. I will get a new SE valve and cable for next week. Once this is installed, I will try as idaho farmer suggests.
Thank you both.

The shop should have caught that and told you about it.

Another example of the incompetence of shops out there. Very willing to take the money but not good at delivering reliable results.

It took a while to get the choke valve and choke assembly delivered from Boats Parts but finally got them. I look forward to go to the marina later this week and install them. Will let you know how it went.

Thanks and have a great day.

Ok,
Looking forward to some good results!

Choke valve and cable installed without problems. The engine runs well, not perfect but well enough to get me to the end of the sailing season. I will then take the motor to the dealership for a final check-up and an oil change. Thanks to you and your good advice.
I had one last question. It concerns a screw on the side of the carburetor that I was wondering what was its use. I tried to upload a picture but that function does not seem to work. A question then for the repair shop.

Thanks again for your help.

Dan

The screw seems to be number 4 on the carburetor diagram.

Well, I sure hope you won't take it back to the place that "serviced" the carburetor last time! They really didn't do you any favors.

That screw is the idle mixture screw. The chromed screw cap is a "false" screw head and is actually covering the head of the idle mix screw. It is intended as a "limiter" to keep you from futzing around with the mixture setting. However, the "cap" is held on to the screw head with glue. Specifically Loc-Tite. It can be removed with heat from a soldering iron but that destroys the cap but it's the only way to adjust the idle mix.

BUT...it is VERY easy to BREAK OFF the head of the mixture screw while removing the cap because the head of the screw is designed to snap off if tampered with. It's just another STOOPID reaction to our worthless political class dabbling in saving us from ourselves. But you couldn't sell your outboard in America without it being on there!

The so called cleaning you paid for probably didn't include taking that screw out and flushing (and back flushing) that idle circuit which is required to obtain a smooth, reliable idle. Shops don't do it because it's not easy to do ( most of them don't know how) and the screw set needs to be replaced because the limiter cap gets destroyed.

You can take the limiter off yourself and, SURPRISE!, the Air Quality Police will be at your door IMMEDIATELY!

Or....maybe not.

Take note of the Oring in that screw set. It will begin leaking additional air (with contaminants) into the idle passage as it gets old. That will eventually FORCE you to take the screw out for cleaning and replacement as the idle quality deteriorates.

The 8D is one fine outboard and should start EZ and purr like a kitten. But that little carb (Honda didn't make it) is the weak link. But with PROPER cleaning and care it can work really well for years too.

Good luck and fair winds.

Question:
When was the last time the valves were adjusted?

Just curious.

Good luck.

Thankfully I did not break the screw cap while playing a little bit with it just to see if it did something to the regime of the engine... I will certainly refrain from trying anything in that area

Not sure the valves were ever adjusted. This is something I will mention to the new technician this Fall.

Thank you again for everything. Not only have you helped me get my motor running again but I have learned a lot in an area where I was totally ignorant. I used to manage an IT department. Never too late to learn.

Ha ha! Glad that I could help "edge-um-cate" you. But I'm totally grateful that I don't need reciprocal help in your field! I'm afraid that I would be a very poor student.

Yes, look into the valve adjustment because, if your main complaint about how she's running now is the idle quality, it may be that a proper valve adjustment might smooth things out.

The valve stems need a small clearance (sometimes refered to as "lash") between the tip and the rocker arm when they are fully closed. As the engine runs the valve head tends to set down and settle into its seat. Just as you would "wear in" your favorite EZ chair. This causes the valve stem clearance to rocker arm to close up. This causes what is called "tight valves". Meaning that, when the valves are "tight" they open too soon and close too late. When the valves get REALLY tight, they can actually be held open a slight amount and that is NOT good. It can eventually lead to engine damage from carbon build up, backfiring and even catastrophic contact between the valve and piston crown.

Hondas are so well constructed, they don't need valve adjustment very often. BUT that doesn't mean that it would be wise to neglect checking them every three or four seasons.

I wish you good fortune with the new tech or shop you use.

One last thing...

The accelerator pump "bellows" (item #11 in the carb parts page link) is subject to deterioration and tearing open. That will immediately allow dirt in the carb. Just give it a look every now and then and replace it asap if that happens.

All fuel....especially gas pump fuel....will contain a small amount of condensate water. Water contains hard minerals in suspension and when it enters the carburetor it will fall out of suspension and plug the tiny passages and orifices of your carb. This is far and away the main cause of carburetor problems and the need for cleaning them. If you only use the outboard on the sailboat minimally, I recommend using canned gas like VP fuel. While much more expensive, it WILL pay you back in less carb issues. Even then though, don't let ANY fuel set around too long. Use it in a car or something rather than letting it go stale.

The BEST way to keep water out is to use fresh fuel from a trusted supplier. It's also a good idea to keep your tank vent closed when not in use.

But, in addition, you need to find the carburetor drain screw and use it. The drain on these goes out a small hose to the fitting next to your water pump tell tale fitting in your stbd. side cover. You can drain the carb into a clear container, let it sit for a bit and inspect for any water in the fuel. Doing this anytime your not going to be using the outboard for a spell will reward you with WAY less carb problems than the next guy.

I should also tell you that the little hose from the water pump tell-tale AND from the carb bowl drain should be replaced every few years. They get hard with time and, even though it's hard to visualize, liquid won't flow as well through them after that happens.

Good luck out there!