BF9.9 (2004) can't operate above 1K RPM and below 3K RPM

Hello Everyone! Looking to try and help my father out with issues we've had with a 2004 model Honda BF9.9. It's on his 16' pontoon he bought a couple years ago. It's never really ran "right", and this Spring we had a local marine shop work on it to no avail so far. I'd rather just work on it ourselves at this point; but I could use some help in what direction to head.

Symptoms the past few Summers has been consistent RPM fluctuation when under way. We generally just use the pontoon for fishing and for slooooow cruising around the lake, around 1,600-1,800RPM is where we like to set it. Problem is it will hold that range for a few minutes, then start "running away" - revving 2K, 2.5K+ RPM, or it will drop down to near idle. I'll increase the throttle a little, only to have it speed up too much, then try to get the RPM back down to our desired range...repeat often...I'm constantly fiddling with the throttle lever fighting the motor speeding up and slowing down in RPM instead of staying steady.

2 weeks ago the shop replaced the fuel pump, and also wanted us to tilt the motor so far that the idle exhaust port was in the water! They claimed the exhaust wasn't exiting the prop correctly and caused cavitation, leading to the erratic operation. Sounded like a screw-ball idea to me. 2 years ago the carb and accelerator pump were rebuilt by the same shop. Last summer I replaced the fuel hose going from the tank to the motor.

Last weekend we had the boat put in the water and it ran around the lake fine on Saturday, but on Sunday we warmed it up, got under way, and within 50 yards it quit and refused to start. I towed it back to our dock with my Seadoo. Took starting ether to get it going again.

On Friday I did some testing...I started it up and let it idle. It idles fine in perpetuity it seems, around 1K RPM. I can also adjust the throttle while in neutral such that it runs at 3K RPM without issue....but as soon as I start to decrease the throttle nothing happens until I have moved it VERY VERY VERY slightly and the motor drops to 1K RPM again. There is ZERO way for me to get it to hold ANY RPM between 1K and 3K while in neutral.

It runs fine at wide open throttle, and it runs fine at idle, it just won't hold a steady RPM where we want to operate it. Any ideas would be appreciated!

Aaron

I'm wondering if you might have a lot of slop in the throttle linkage somewhere?

Is this a Honda control head? Or maybe something that's been "rigged"?

Alan Hicks said:

I'm wondering if you might have a lot of slop in the throttle linkage somewhere?

Is this a Honda control head? Or maybe something that's been "rigged"?

Click to expand...

Thanks for the reply!

There doesn't seem to be any slop in the throttle linkage that I can tell, and I suspect that it is not a Honda control head; not sure where I would look to tell. I'd be happy to check if there is a way to do so. The pontoon is an Odyssey as an FYI.

One of the things I check was to see if there was any play in the linkage at the carb and throttle cable and it seemed to be solid. Not sure if this is another symptom or if I was off on my assumption, but there is a screw with a spring on the right side of the carb between it and the motor. It appears to be the throttle stop/idle adjustment, based on what I can see and from prior experience with carbs (I race motocross, so I know my way around a 2 stroke and carbs on those pretty well). The motor tends to idle fine, but when cutting power and switching from forward to reverse or reverse to forward it can go so low that it stalls. It starts right up, but to prevent that I adjusted that screw last summer to get the idle to 1,100 or so. It cut out on me once already when pulling into the dock so I tried to adjust that again and it made no difference. It was only 2 turns out as it was, and when I tried to adjust it in farther it just bottomed out and made no difference in the idle. I turned it back out just in case I wasn't seeing things right and it wasn't the throttle stop.

At a loss really...could it be electronic? Advance isn't right? Bad sensor somewhere? Doesn't seem like there is that much electronics on it based on the owners manual and electrical diagram...

These are very simple, straightforward outboards and you are correct, not any electronics except the CDI unit that provides ignition switching and timing and also monitors engine temp and oil pressure senders to provide engine protection.

You are also correct about the "curb idle" screw or, as you say "throttle stop". Not much you can mess up there other than to cause harsh gear changes due to high idle. These are intended to idle @ 900 rpm.

Having said that, I think that you have a carburetor problem and you are ALSO correct to step away from the shop you've been using.

The carburetor has three basic "modes" of operation:
1. Idle
2.TRANSITION
3. High speed.

I capitalized TRANSITION because it seems something is going on with that "mode".
The transition circuit in the carb is what you are relying on to CONSISTENTLY flow fuel for the intermediate speed you want.

In addition to some very tiny holes near the throttle plate at the top of the carb throat, this carb uses high speed and intermediate jets to deliver fuel. But, there are a couple of other things to consider. The biggest is AIR FLOW. There are air bleed (or feed) circuits that help "siphon" the fuel up into the delivery passages and out those tiny holes.

You mentioned the accelerator pump. Unique to these carbs is the fact that even have one.And,.it's probably the WORST accelerator pump system I've ever encountered in many years of carburetor work. BUT...it is VERY effective at causing "runability" problems if, even in the slightest, restricted.

I believe that the shop didn't do a THOROUGH job cleaning your carb. Few shops...or individuals... actually do.

I also believe that pretty much anyone that can read, use a cell phone camera as well as a screwdriver CAN repair and clean one of these IF (and here's where I have to say I received 0 compensation from Honda nor anyone else) you have in your possession the

HONDA MARINE CARBURETION MANUAL

about $40 + shipping.

Get it, read it, fix that little carb.

Pay particular attention to the brass tube that protrudes into the front of the carburetor's throat. That is the accelerator pump nozzle tube. It should shoot a stream of pressurized brake or carb cleaner several FEET out the rear of the carburetor with the throttle propped wide open.
If it won't deliver 6 to 9 feet of a stream, there's probably no use in reinstalling the carb.

Also, the tiny holes at the top of the carb throat. The book explains them and their function in detail. What it doesn't say is BACKFLUSH each one using the same can and delivery straw you choose to buy your cleaner in. Forward back, forward back, forward back for each circuit that the book points to.

I have used several cans of cleaner to get badly plugged carbs cleaned. The good news is I've not found one yet that couldn't be cleaned.

Or. If you prefer, bolt on a new carb for less than $200.

Good luck.

Geez, nicely done. Agree with every word, was thinking carb cleaning, but a little shy replying as I have no experience on these little guys.

Thanks Alan. Honda carbs are ok but you know that they are very sensitive to deposits in them due to fuel contaminants. Mostly water.

I forgot to mention the importance of draining the float chamber ANYTIME the outboard isn't used for more than a couple of weeks.

NO DRAIN MORE PAIN!

jgmo said:

These are very simple, straightforward outboards and you are correct, not any electronics except the CDI unit that provides ignition switching and timing and also monitors engine temp and oil pressure senders to provide engine protection.

You are also correct about the "curb idle" screw or, as you say "throttle stop". Not much you can mess up there other than to cause harsh gear changes due to high idle. These are intended to idle @ 900 rpm.

Having said that, I think that you have a carburetor problem and you are ALSO correct to step away from the shop you've been using.

The carburetor has three basic "modes" of operation:
1. Idle
2.TRANSITION
3. High speed.

I capitalized TRANSITION because it seems something is going on with that "mode".
The transition circuit in the carb is what you are relying on to CONSISTENTLY flow fuel for the intermediate speed you want.

In addition to some very tiny holes near the throttle plate at the top of the carb throat, this carb uses high speed and intermediate jets to deliver fuel. But, there are a couple of other things to consider. The biggest is AIR FLOW. There are air bleed (or feed) circuits that help "siphon" the fuel up into the delivery passages and out those tiny holes.

You mentioned the accelerator pump. Unique to these carbs is the fact that even have one.And,.it's probably the WORST accelerator pump system I've ever encountered in many years of carburetor work. BUT...it is VERY effective at causing "runability" problems if, even in the slightest, restricted.

I believe that the shop didn't do a THOROUGH job cleaning your carb. Few shops...or individuals... actually do.

I also believe that pretty much anyone that can read, use a cell phone camera as well as a screwdriver CAN repair and clean one of these IF (and here's where I have to say I received 0 compensation from Honda nor anyone else) you have in your possession the

HONDA MARINE CARBURETION MANUAL

about $40 + shipping.

Get it, read it, fix that little carb.

Pay particular attention to the brass tube that protrudes into the front of the carburetor's throat. That is the accelerator pump nozzle tube. It should shoot a stream of pressurized brake or carb cleaner several FEET out the rear of the carburetor with the throttle propped wide open.
If it won't deliver 6 to 9 feet of a stream, there's probably no use in reinstalling the carb.

Also, the tiny holes at the top of the carb throat. The book explains them and their function in detail. What it doesn't say is BACKFLUSH each one using the same can and delivery straw you choose to buy your cleaner in. Forward back, forward back, forward back for each circuit that the book points to.

I have used several cans of cleaner to get badly plugged carbs cleaned. The good news is I've not found one yet that couldn't be cleaned.

Or. If you prefer, bolt on a new carb for less than $200.

Good luck.

Click to expand...

Thanks for the feedback! Unfortunately the pontoon is at the shop that did the "work" at the moment. My dad told them to go ahead and look at it again (though I would rather have done more myself). I would be willing to bet that they just rebuilt the accelerator pump back 2 years ago and didn't do anything else. The exterior seal on the pump shaft was leaking so it obviously needed a rebuild, but how far they went I have no way of knowing of course.

I'm fine with cleaning and rebuilding the carb that is there; I've cleaned and rebuilt plenty of carbs. Just cleaned my Seadoo GTI carb this Spring in fact, and rebuilt it a few seasons ago now. Happen to have any links to manuals and rebuild kits? Or should I use the search feature here or in Google?

The pontoon gets used a few times a week usually in the Summer. My parents have a cottage on an inland lake in MI. That cottage is about 5 minutes from their house so they'll frequently head out there on evenings for a cruise around the lake. Then on Thurs-Sun I am there with the wife and kids so we use it all weekend. It doesn't use much fuel so while it gets fuel circulating through it often, the gas itself is not getting replaced often, though we do use a marine fuel stabilizer. For the PWC I go through 5-10 gallons a weekend so I don't bother with stabilizer until late in the season.

https://publications.powerequipment.honda.com/marine

Type Carburetion into the search box in the link above if you want to buy the rebuild manual. It is well worth the money.

The site boats.net has all the parts you need at the best prices.
There are no rebuild kits per se' for these carbs.

But you will likely only want to buy the gasket set and, possibly, the "jet set" tube as they typically crack after a time.

If the accelerator pump inlet check ball won't stay put, you would need a new float chamber but I wouldn't bother. You could throw the pump piston away and probably never notice the difference as that accelerator pump is not much use anyway.

Use new mount gaskets and inspect the spacer for cracking. You can glue or silicone the edge if it is. No need to buy a new one.

Good luck.

Ok, got some new info...

Boat has been at the shop until today. Said they found an air leak after the carb they had to fix. Didn’t help. They replaced the sending unit for the automatic choke. Didn’t help. Tore into the carb and said it seemed like we had been using fuel with ethanol in it and gas smelled bad. We run 100% gas at 91 octane (it’s available down the road at the local gas station), though 91 is more than this needs...it’s just the only 100% gas option. Said they found the seal/oring for the choke assembly was rotten and had to replace it. Today they called my dad...no change. They couldn’t figure it out. I told my dad have them put it in the water and I’ll take a look.

Got it back and I took it around the lake. Farthest point away it just quit at 3/4 throttle. Wouldn’t start. I could get it to sputter, but it would die almost immediately. After trying a few times it would stay running, but idled at 4K! It refused to idle down, and eventually I just cycled the key off and waited for the tach to drop to around 1k and flipped the key back to run and quickly got it in gear. Got a little ways and same thing. I finally got it restarted and back to the dock, took off the cover and started inspecting.

I noticed they replaced the fuel line from the filter to the fuel pump with a translucent yellow one. It was SUPER long and they looped it down under the oil filter and over to the pump. Fired it up and noticed TONS of air in the line. Constantly a 50/50 amount of air or fuel hitting the fuel pump! I took everything out from the pump to the quick disconnect on the motor housing. I routed the fuel line from pump to filter over the oil filter and between the carb so it has a direct straight line from the pump to the filter and nothing will interfere with it. It won’t interfere with anything else either. I checked the fuel filter to see if it was leaking by just putting my finger over the outlet and blowing into the inlet. Held fine. Since I had extra fuel line I replaced the black line from the filter to the quick disconnect as well so I could check for air. Put it all back together and fired it up. Idled at 3K and wouldn’t idle down. No air in the line now though! Solid fuel. I remembered that the idle adjustment screw was all the way in because I previously tried to adjust it up. So I backed it out. Idle dropped immediately. I turned it out almost 3 turns until it was puttering at 1K happily.

No wonder if ran so inconsistent! They replaced the fuel pump first thing this year and must have done the fuel line at the same time. I did notice during lake test that I would still get the occasional air bubble before the fuel filter...so I bypassed the quick disconnect entirely and have the line from the tank going direct to the fuel filter now. Zero air bubbles now.

It runs best it probably ever has...but I still need to fiddle with the throttle a lot. 1400-1600 RPM it seems happy, above 2400 it seems happy, but in between it will run at a given RPM for a short bit, then idle down or “run away”.

Should I look to carb at this point now that I unscrewed up what the “mechanics” did? I’m completely comfortable with working on carbs, I’ve raced motocross for 3 decades Which comes with lots of carb work,and have Seadoos that I’ve done rebuilds on as well...But...I noticed that between the manual and buying parts, plus the trial and error time I can quickly surpass the $165 for a new carb. So if it sounds like a carb problem I’m inclined to just buy new so I have a good platform to start from. The existing one has been into before our current mechanic worked on it, so who knows what’s been done.

sorry for the lengthy post but I wanted to paint the entire picture. Guidance much appreciated!

Good onya mate fer takin' the bull by the horns!

That "shop" has been wasting your time AND your money!

My opinion is that the new carb will make your outboard AND you happy.

BUT....

Before you pull the trigger on that, try spraying some carb cleaner at the carb to manifold interface and around the intake mounting flange while you run her at idle to check for vacuum leaks. If you find any, seal them up.

Good luck.

jgmo said:

Good onya mate fer takin' the bull by the horns!

That "shop" has been wasting your time AND your money!

My opinion is that the new carb will make your outboard AND you happy.

BUT....

Before you pull the trigger on that, try spraying some carb cleaner at the carb to manifold interface and around the intake mounting flange while you run her at idle to check for vacuum leaks. If you find any, seal them up.

Good luck.

Click to expand...

Appreciate the feedback. I’ll give it a shot tomorrow and see what I find. I see one of the gaskets is new between the carb and the spacer, but spacer to manifold is old. Since the manifold and head look like they are 1 piece the only leak should be the place they supposedly found right where the spacer and gaskets are. Since they supposedly “fixed” the air leak I suppose I better double check. I’ve ridden dirt bikes and fiddled with stuff that had air leaks and lean conditions before and this doesn’t behave like any of that...but then again marine stuff seems to be just a bit different...

Yeah, I wish my dad hadn’t sent it back to them. I had some pretty strong words with them when it came to my dad this year in worse shape than before they “fixed it”. They told him they’d take care of it so he gave them another shot. No more.

If I didn’t live an hour away and Summer wasn’t so short I’d spend the time to really dig in, but both those facts conspire to make it potentially a “better” route to have someone else work on it. That was the assumption at least. No more. I’d rather dig in and really find the problem with the carb and fix it, but being in MI the Summer is already half over and we just now got it back. I don’t want to waste more time fiddling with it to save a few bucks. So like I said, new carb just might make the most sense since the one that’s on there now has also been messed with a bunch before. Starting from a known state can only be good...

I’ll try the carb cleaner tomorrow and report back.

thanks for all the feedback.