Do I have a BF20 or a BF9.9?

I just took ownership of a used boat from a family member. I put it in the water and started it to realize the cooling indication was not squirting water. I took the screens off the water intake to find the blades of the impeller. I took off the lower unit to find not much left of the impeller. So, I went on the quest to order parts. I found the serial number plate and it has this 'BAMJ-1400927' and 'BF20'. I found this odd since the engine cover says 9.9. I called the dealer today to find the water pump rebuild kit and read them the serial number. They said you have a BF20 and I said, but the cover says 9.9. We were both confused. The water pump kit for the BF20 fit fine. Today I took it for a cruise. It surely feels like a 9.9, could barely go 7 MPH on a fiberglass 14' twin vee.

So the question is, do I have a detuned BF20? and if so, is it possible to tune it to 20HP?

Thank you

Looks to me to be about a 2007 BF20D.

Not sure about the performance as fiberglass hulls can be very heavy depending on manufacturer and build date.
If you want to "tune it up" you need to know that the carburetor on these can be problematic if not taken care of properly.

These carbs get mineral scale built up in their tiny passages. However, that typically manifests into hard or no start. You don't mention hard start. But...be aware that you NEED to drain the fuel from the float chamber any time you won't be using the outboard for more than 2 or three weeks. Carb draining keeps these carbs working tip-top almost indefinitely. Find the drain screw and use it.

Also, this engine comes equipped with a black, rubber looking thingy attached to the engine oil dipstick called a "fuel chamber". (The 9.9 didn't have this device)
The fuel chamber acts as a water trap even though that wasn't Honda's reason for putting it on the engine. It needs to be removed periodically and completely drained of all liquid. This too can cause starting and running issues if the water leve in itl gets too high.

Use a spritz of WD40 on the dipstick tube to aid in removing and be careful not to break the tube by being too rough.

Beyond those items would be a cylinder dropout test to make sure both cylinders are contributing evenly. The engines can run amazingly smooth on only one cylinder and many people can't discern the difference.

Unplug each plug wire and reinstall to make doing it while the engine is running much easier and quick. Like the dipstick tube, a little lube helps here. Only this time use a small amount of dielectric tune up grease smeared around the inside of the sparkplug boot.


Start the engine and increase speed to about 500 rpm above idle. Using a pair of DRY gloves or a DRY shop towel, remove each plug wire momentarily, one at a time and note any changes in how the engine runs. If it stalls immediately with one cylinder not firing, you have a weak or non running cylinder in it's companion.

After that bit of research you can explore engine valve timing and valve adjustment. It is not uncommon to find that these engines have never had a valve lash adjustment done on them.

Good luck and ask as many questions as you like.

Thank you for the detailed response.

My biggest question is why does the cover say 9.9 and is there any other way I can verify it's a 20 or a 9.9? I'll check the dipstick and can post a photo later.

It runs well, starts reasonably quickly and idles well too. I came with a servo attached to the throttle linkage on the carb and a controller on the console to control the throttle for low speed trolling.

Yes the boat is heavy, not sure the weight but it's a 14' twin vee and quite a bit of gear. Minn Kota RipTide trolling motor, two electric down riggers, fish finder and so on. I will be taking some of the gear off. I would expect more than 7mph out of 20 HP, but it feels more like a 10 HP on the water. My 15' alum boat with 8hp goes 12mph. I don't think it was abused but not taken care of very well based on the water pump condition. I'm sure the oil was changed but prob not much more.

I'll pull the plug wires and see how it behaves. It does not seem to get to max RPM, it stops at about 4800.

The explanation for the cover could be that it's simply a replacement for a lost or damaged one. I do think the covers, or "hoods" will interchange on the 9, 10, 15 and 20 "D" outboards although I'm not positive.

Or, the previous owner wanted to use the rig on a "Under 10hp" impoundment. The very reason 9.9hp outboards exist in the first place. It would be simple and cheap to remove the 20 decals and overlay them with 9.9 decals to gain access to these restricted reservoirs.

On an outboard this old you never know what may have been done.

Yes, it sounds down on power so, if it passes the cylinder contribution test, timing and valve adjustment might be next.

Although...I might be suspicious of how that throttle servo was installed and would first verify that the engine is actually achieving full throttle.

Looking forward to your photo of the dipstick tube.

I performed the cylinder check by pulling the plug wires one at a time. The first time it stalled but I think the RPM was a little low, just above 1k. I did it again with slightly higher RPM. It kept running on each cylinder but a tad rougher as I would expect. I forgot to get a photo of the dipstick.

Here's a video of WOT. https://youtu.be/acVXOEGTm4k

Next I will check if the servo is stopping the throttle from opening up all of the way.

It sounds like the test came out normal.
The video does look like it's running normally and at full or near full throttle.
So, I see why you're interested in the HP rating.
But that is a heavy hull loaded with gear.

Another consideration for pushing the boat efficiently is the placement of the outboard on the stern and the propeller. Do you know the diameter and pitch of your prop?

Is the outboard hung on the transom so that the cavitation plate is even with the hull bottom?

I only ask these questions for reference. I'm not convinced that there's anything wrong with your setup but these are basics that sometimes get overlooked.

I would still be interested in the dipstick photo.

I took a look and the servo is not blocking throttle operation, however it seems the throttle could open a little bit more when it's set at wide open on the remote.

See attached photo of dipstick.

Since it's a twin hull there is no bottom of the hull where the cavitation plate is set.

The cav plate still needs to be situated "in relation" to the hull bottom. Doesn't matter how many hulls you have.
Too shallow=no "bite". Too deep=drag. A long shaft outboard on a shallow draft boat = excessive exhaust back pressure + drag.

Is there a black, rubber thing about 3 inches long × 2 inches in diameter mounted on the dipstick TUBE? It would have fuel lines connected to it from the fuel pump and fuel lines going to the carburetor. The close up of the oil dipstick doesn't show the TUBE.

I've included photos of the prop (9-1/4x10") and motor mounting in relation to the hull. I'm not sure I see what you're asking for with regards to the dipstick tube. I think I'd need to take the lower cover off to see that. I was thinking of ordering a 9-1/4x8" prop to see if that helps with a heavier underpowered boat. I've also taken about roughly 100 lbs of weight from the boat. Yesterday I was able to reach 7.5 mph, but the motor cannot reach max rpm range. It's stuck right around 5k RPM.

Thanks for you help.

Yes, taking the side cover off is necessary to access the fuel chamber. But I do think I can see the top of it in the photo. That would answer your horsepower question.

I can't make an accurate assessment of the outboard height based on the pic but it does seem to be mounted a tad to the low side. I wouldn't sweat that though because it's not ridiculously off if even off at all.
If that is a jackplate that it's mounted to (can't tell from here) then you could experiment with height adjustment using that feature.

I do think that an 8in. diameter prop will help with rpm increase and I might even try a 3 blade if one is available. BUT...it's a BIG BUT...while the 3 blade might aid in increasing rpm and top speed, it may disappoint in getting this heavy hull moving as opposed to the 4 blade. I am by NO MEANS a "prop upgrade selection guru".

It's such a guessing game and props are so expensive that finding one that's ideal for the application can be difficult. Having a spare prop on board is always a good thing though if you can afford it.

Try asking this question at a couple of boat repair shops and see what kind of feedback you can get before spending the money.

Good luck.

Thanks. Here's a few more photos. It think it's a few inches too low. Moving it would require drilling a few new holes and filling old ones. I'm not sure I'm there yet. I'll try a new prop first.

Well...It does look a bit low but I don't think that it's so far off of "optimum" tha that it's worth punching new holes. After doing that you might just be very disappointed with the results.

One bummer about these outboards is the weak trim pump. They don't have enough power to enable you to trim the outboard UP when under full throttle. But...you can trim DOWN easily when powered. Have you played around with the motor trim to see if you can get the hull to plane? Trim it up a few degrees, take off and then trim down a degree at a time to see what it does.

This boat should probably go faster than it does with this outboard and a new prop to bring the rpm up is likely to help. But you will probably find that this heavy hull needs 40 or 50 hp to efficiently pop it up and plane.

Good luck.

I did try to trim without much success. I was able to locate a 9-1/4-7" 3 blade prop but I have not ordered it yet.

I was able to confirm that it has the fuel chamber attached to the dipstick tube.

One thing I noticed is that if I put the throttle wide open while not running and go back to the engine, I can open the throttle slightly more by hand, maybe an 1/8" more. Do you think I should spend anytime adjusting the linkage to get it to fully open?

Oh yes! Try to get whatever adjustment you can to make sure that the throttle arm opens the plate all the way.
This carb, like pretty much any carb, uses air flow through the venturi to pull fuel up out of the float chamber. Pretty much like drinking with a straw. Any lack of the plate being fully open is directly related to less air flow and less air flow means less fuel is sucked up.

Just be careful not to put the linkage in a bind as that would probably lead to other problems.
It's a take what you can get sort of thing.

Also, remove that fuel chamber and shake out all the liquid. If water is built up in there it can starve the carb for gas. If you pour the contents in a clear container and then let it sit for a few minutes, you will be able to see how much water may be present after it separates.
This is a routine maintenance chore needed on this system.

Spray WD-40 in the dipstick tube and holder ring to help it easily slide up and off and back on.
Be gentle with the tube! They can bend or snap off if forced and you don't want to have to replace it because it ain't easy.

Have you shopped ebay and craigslist and Facebook for a prop? I hate to see someone paying retail for an experiment that doesn't work out if they don't have to.

At least now you know it's a 20hp. Those are fairly desirable outboards and would bring a much better price than the 9.9 of the same year and condition if you decide you just want to repower with a larger outboard.

Good luck and let me know how you make out.

So, I was able to adjust the linkage to get the plate to open all the way. My butt dyno tells me I gained a tiny amount of something, but not much to make a difference. top speed with two people topped out at 7.8, close to what I could manage alone.

I was unable to remove the chamber. I need wd-40 and more time to get the hoses off.

I'll order a prop tonight for $80 shipped, not terrible, so if it does nothing I won't be too heart broken. I'll update once installed.

I did a lot of reasearch on props, and had it pointed out to me in several videos the difference between a forged prop and one where they weld the blades on can make a big difference in performance itself. I don’t claim to be real knowledgeable about props at all, because I’m just learning myself. I just wanted to share what I gleaned on the you tube videos and encourage you to look into it further.

question.... at 7.8 mph is the boat actually getting on plane yet?

Thanks, I ordered an OEM Honda prop, not sure how it's manufactured but at 20HP, I don't think I'll see much difference between metals and manufacturing process.

No, it's not even close to being on plane. It's very underpowered for a heavy boat, so I'm hoping to get the engine operating near max rpm and max power and maybe 10+ mph top speed or so. I went with a 7" pitch since I'm barely at 5000 RPM and should be closer to 6000 max rpm. I've been removing as much weight as possible from the boat. The family member I bought the boat from did not want speed and loaded it down with lots of gear.

kirkll said:

I did a lot of reasearch on props, and had it pointed out to me in several videos the difference between a forged prop and one where they weld the blades on can make a big difference in performance itself. I don’t claim to be real knowledgeable about props at all, because I’m just learning myself. I just wanted to share what I gleaned on the you tube videos and encourage you to look into it further.

question.... at 7.8 mph is the boat actually getting on plane yet?

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Did you order the 3 blade? I believe those are going to be more efficient..... I'm still learning about how this stuff works myself. I just bout a 19' fiberglass boat with a 4.3 V6 mercruiser. You talk about heavy? Mine is about 2800 empty.

I'm trying to get a trolling motor matched up to this thing. I've got a newer 8hp Honda short shaft that is a wonderful motor. It pushed my 14' low profile aluminum boat 22 mph with just me and my fishing gear and 18 mph with two of us. I put a set of dolphins on the cavitation plate to help get it up on plane quicker.

But putting this baby on my monster boat here has been disappointing. Its got too much free board at the stern requiring a adjustable mounting plate to lower it and raise it out of the water. The motor has plenty of power for trolling in still water, but with wind and current it doesn't do well at all. That and this short shaft is killing me. trying to steer this thing requires sitting on the back rail with the tiller handle at 35-40 degrees. I think i'm going to keep this motor for my little aluminum and buy a log shaft motor to eliminate my tiller issue.... But the funny thing is, when i talked to my Honda dealer he said a 9.9 wont power that bigger boat any better than an 8hp like i have. THAT ....i don't understand.

But.....like i said earlier... i'm still learning about this stuff myself. i wish you luck on your new prop. Kirk

New Prop came today 9.25x7" I'll post an update later today on how it impacts performance. I'm hoping I can get on plane even if I only go a few mph faster. I think getting the engine up past 5k RPM will allow it to develop close to full power and push the boat out of the hole. fingers crossed.

I'll be tuned in.
Good luck.

So, not what I expected. It now bounces off the rev limiter and goes 1mph slower. It is cavitating a lot. It can't seem to bite the water. I'm at a loss. what about the fins that go on the cavitation plate, would those help? https://youtu.be/5_mGy6-D8cE

or it's ventilation I'm not sure which.

Well, I don't think the dolphin fins are going to help you a whole bunch in this case. They do a great job of helping a marginally powered boat get out of the hole but I'm just afraid that the 20 isn't going to ever provide enough oomph for that hull. I think a 40 or 50 would be a much better match.
But now that you've payed for the prop the cost of the fins isn't great and they are too easy to install not to try. In other words, they can't hurt.

Try to get any load as far forward as possible.

Good luck.

Hello there,

I know a bit of TVs and Carolina skiffs and i'm almost positive those hulls are rated for 40-60 HP and in the upper side being ideal. You might be better off finding a used 40-60 and sell off the motor you have, before you keep dumping money on a 20.

thank you. I agree. I will try the $20 dolphin fins. I don't expect it to be fast but if the prop could just bite it would go a lot better. I'll look for a 50. Any suggestions where to find one?

Mr Main22 said:

Hello there,

I know a bit of TVs and Carolina skiffs and i'm almost positive those hulls are rated for 40-60 HP and in the upper side being ideal. You might be better off finding a used 40-60 and sell off the motor you have, before you keep dumping money on a 20.

Click to expand...

skcihs said:

thank you. I agree. I will try the $20 dolphin fins. I don't expect it to be fast but if the prop could just bite it would go a lot better. I'll look for a 50. Any suggestions where to find one?

Click to expand...

I would go ahead and give a call to your local outboard marine shops and they usually have a couple used outboard available and very reasonably priced for these low powered outboards. You start seeing a skew once you get into the 90+ HP range.

Research a couple options i'm sure you can find something. You'd be set after and be able to run a lot faster and farther distances. Good Luck