BF90 (Carb) stalls under fast load

[totallydave]

Hi,

I have a power loss during acceleration issue with my 2005 (carb version) Honda BF90 on my Zodiac Pro Open 550 Rib. I have had the issue for quite a while (pretty much since having the boat) and have mentioned it when having the engine serviced several times, but no-one can find any specific issue with the engine – but they never really test it under load.

The issue is that if I open the throttle hard from standstill, the boat stalls. If I push gently through the rev range it picks up, you feel a minor delay, then it starts picking up again – after this, it runs straight up to WOT without a problem.

I have lived with the issue for a while, but with the kids getting more into waterskiing and wakeboarding, it means I have to drag them a long for a bit before pulling them out. The issue does seem slightly worse with more people on the boat. Once through this issue, the boat performs well, quickly onto the plane and have seen 35 Knts from her on flat water.

I am no expert on anything engine or prop related, so looking for some general advice / pointers to think / try. As I have had several ‘all clear’s on the engine, I am wondering if it could be prop and / or fuel intake related. Looking for advice on what to look for / try.

Many thanks,

David

[Peter_C]

I am not familiar with those carbs, but if it was a vehicle carb I would fault the accelerator pump. The accelerator pump gives a shot of fuel into the carb as you open the throttle.

Tear the carbs down and inspect them. Clean the orifices with carb cleaner and a blow gun. Put a rebuild kit into them if needed. Very carefully inspect the accelerator diaphragms.

If it runs great at all other times, except when whacking the throttle open, it should be a fuel issue.

What it is most likely NOT:
Timing
Fuel pump
Float level
Vacuum leak
Compression/Major mechanical issue
Anything to do with the lower unit/Prop

[9D280]

I guess you have a mild version of what I had - your idle jet is running slightly too lean.

(I'll apologise to the engine techs now for the "non tech" way I write this)

Your carb has essentially got two ways to get the fuel into the air stream - the idle jet (adjustable) and the main jet(s), which are fixed. As you open the throttle the pressures (or more accurately, vaccum) inside the carb cause the fuel to flow through the main jet at an appropiriate flow rate. Problem is if you whack the throttle open too quickly the initial slug of fuel can't accelerate out the main jet quick enough, so it stalls.

By making the idle mix slightly richer (slighty more fuel per volume of air) you have a "fuel cushion" to get you through that initial movement of the main jet fuel.



I had a Suz 25, a Yam 55 and my Merc all suffered this when I up pitched the props (and so put slightly more load on the engine at idle).


The way to test if that's your problem - bang the throttle open, and as you do so, blip the choke. (That gives you an extra wee slug of fuel when you need it). If it takes off like a scalded cat, richen your idle mix by about 1/8 of a turn* on each carb.


* 1/8 turn is a guess!

[totallydave]

Thanks Peter. I'm not too confident in stripping down the engine, so may have to reserve this for a discussion with the guys that do my annual service.

I do agree it sounds like fuel. I did wonder about switching to an aux tank just to prove there were no issues on the fairly long feed (and water filter) from the main tank.

[totallydave]

Thanks for this, it sounds like something I can try quite easily. Again, not very confident about twiddling with things inside the engine but this does sound easier than carb strip-downs.

[Peter_C]

Originally Posted by totallydave

I did wonder about switching to an aux tank just to prove there were no issues on the fairly long feed (and water filter) from the main tank.

Won't do a thing for it. If it were a fuel flow problem it would show up at full throttle. The carb bowls themselves hold enough fuel to run it for around 10 seconds at full throttle anyhow.

Enriching the mixture might do it. If it idles smooth though...they are probably adjusted correctly.

Do this, take the carb anti backfire cover off so you can see down the throats. Make sure there is fuel in the carbs, and while looking down the throat with a flashlight have someone whack the throttle open a couple of times. You should see a shot of fuel into each carb (Assuming it uses an accelerator pump). If there are differences between the carbs then there is an issue.

A good visual inspection can tell you a lot. Look everything over carefully.

[9D280]

Dave,

I have not mucked about with Honda carbs, but if you can post your engine's year I'll see if I can find a pic or exploded diagram to tell you what screw to turn.

Have a read of your manual - that sort of adjustment might be in there.... if it doesn't say "take me to your dealer" for everything except adding fuel!

[headdamage]

I had a honda BF45 that would do this. Changing the broken thermastat helped but didn't completely fix it.

[totallydave]

Originally Posted by Peter_C

Do this, take the carb anti backfire cover off so you can see down the throats. Make sure there is fuel in the carbs, and while looking down the throat with a flashlight have someone whack the throttle open a couple of times.

Thanks again Peter, but I wouldn't know an anti backfire cover from my elbow. Perhaps I need to source a service manual and start educating myself a little more.

[totallydave]

Originally Posted by 9D280

Dave,

post your engine's year

It is a 2004 BF90A.

Thanks. I never got a manual when I purchased the boat S/H. Based on other posts I am considering getting a service manual and trying to learn a bit more. I a only feel qualified to 'add fuel' and 'take to dealer' at the moment.

[Nos4r2]

To be fair on your abilities, this is a problem that unless you've got a full workshop and more knowledge than you profess, you really should take to a (different) dealer.

[9D280]

Nos,

If it is just an idle mix problem all he needs is a screwdriver and a stretch of water to run the boat!


2004 BF90A4 JHTA Honda Outboard CARBURETOR Diagram and Parts

I think the idle jet is item 5 on that one.

What I can't tell is whether a CW turn is open or close. If it were me, I'd go CCW first, but that may be wrong.


As you don't sound like you are 100% certain, I'll puit this in basic steps. Apologies if I'm telling you old news.


- Do the "chok blip" test. Assuming it doesn't continue to stall and/ or throws you off the back of the seat when you do that test:

- take a digi pic or note of the screws' positions (so you can put them back in the same place if it doesn't work) and rememebr how far you have turned the screws so you can put it back.
- turn each of them 1/8 turn CCW. (1 per carb)
- test it.
--If it totally stalls or doesn't even start, turn the screws back to where you started, then turn them 1/8 turn CW & test again.
-- if it works, put the lid back on.

- if it seems to work a bit better, but still not brilliant (whichever way you end up turning the screws) , repeat with another 1/8 of a turn & continue to do so 'till it stops stalling.

- if you then want to be really anal about it, you can then turn the screws back 1/16 turn at a time 'till it stalls or hesitates and keep adjusting to & fro until you get an optimal setting.

Hopefully that is clear enough. The only thing I would say is I don't know these carbs, so if yours looks nothing like the pic, or you (understanably) don't want to trust my guess as to screw 5, hopefully someone will be along soon who knows that engine better than I do and can confirm / correct my guess.


The setting process still stands even if it is another screw!

[CDK1]

Originally Posted by totallydave

It is a 2004 BF90A.

Thanks. I never got a manual when I purchased the boat S/H. Based on other posts I am considering getting a service manual and trying to learn a bit more. I a only feel qualified to 'add fuel' and 'take to dealer' at the moment.

The complete service manual will be available on this forum soon.
I was owner of an identical engine for less than 2 weeks and encountered so many problems I immediately bought the manual, then had the opportunity to return the whole rib to the previous owner.
By now I have a Yam F100DET with electronic injection....

[9D280]

Originally Posted by CDK1

The complete service manual will be available on this forum soon.

I'm not on the admin team, but be careful you don't breach any copyright rules!

Boatinfo - The library, containing manuals, brochures and other technical documents from the last hundred years!

has a lot of info......

[CDK1]

Copyright should apply to fiction only.
IMHO the customer who wants to service or repair a product should have unrestricted access to all relevant information.

[9D280]

I'm not so sure on that..... Copyright might not be the correct legal term, but posting someone's intellectual property without permission usually ends badly. Someone with more knowledge on the matter will no doubt chip in later...



I agree with your sentiment, but the OEM may have other opinions.


How many microwaves say "no user servicable parts inside"? Utter b*ll*cks, but it's safer all round doing that than finding lots of dead numpties fried by an HV capacator.

[totallydave]

Originally Posted by 9D280

2004 BF90A4 JHTA Honda Outboard CARBURETOR Diagram and Parts

I think the idle jet is item 5 on that one.

Thanks soo much for all the info and time you have taken to find diagrams, etc. I really appreciate it. But the diagram itself puts me into a bit of a cold sweat.

In a previous trip to a service agent, I had the carbs 'balanced' (but not cleaned). I watched this and saw that he adjusted 3 screws (one cyl was 'base set' and did not have a screw). Not sure if this is the same thing or would have involved the mixture?

I looked at the documentation on boatinfo (not bad but poor copies) but could not see any specific reference to a mixture screw.

I get the whole point of take a note of current settings and return them if it doesn't work, and I am comfortable with this concept but unless it is 'bleeding obvious' where the mixture screw(s) are, I am probably erring on Nos' advice of take to someone who does.

[CDK1]

Originally Posted by 9D280

How many microwaves say "no user servicable parts inside"? Utter b*ll*cks, but it's safer all round doing that than finding lots of dead numpties fried by an HV capacator.

A slight deviation from the topic, but one of my favorites!
The uWave innards are indeed not serviceable for the average user. But from a working one, the magnetron, capacitor, resistor and transformer plus a piece of aluminum tube of the right size you can turn into a silent, deadly weapon to kill annoying electronics like "ghetto blasters", cell phones and car radios in a recreational area. I've even used it to instantly obliterate weeds in my garden.

[tworotorturbo]

I had the same issue on my BF90. It ran excellent but would stall when given instant WOT. The tiny jets got clogged. Its a fairly common minor issue.

[HughN]

Originally Posted by CDK1

A slight deviation from the topic, but one of my favorites!
The uWave innards are indeed not serviceable for the average user. But from a working one, the magnetron, capacitor, resistor and transformer plus a piece of aluminum tube of the right size you can turn into a silent, deadly weapon to kill annoying electronics like "ghetto blasters", cell phones and car radios in a recreational area. I've even used it to instantly obliterate weeds in my garden.

Ha! I've often thought about doing that!