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Old 26 August 2014, 03:09   #1
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BF90 (Carb) stalls under fast load

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
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Old 26 August 2014, 11:29   #2
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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
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Old 26 August 2014, 11:48   #3
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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!
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Old 26 August 2014, 12:33   #4
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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.
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Old 26 August 2014, 12:36   #5
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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.
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Old 26 August 2014, 13:21   #6
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Quote:
Originally Posted by totallydave View Post
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.
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Old 26 August 2014, 16:56   #7
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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!
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Old 26 August 2014, 17:31   #8
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I had a honda BF45 that would do this. Changing the broken thermastat helped but didn't completely fix it.
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Old 27 August 2014, 02:57   #9
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Quote:
Originally Posted by Peter_C View Post

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.
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Old 27 August 2014, 03:05   #10
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Quote:
Originally Posted by 9D280 View Post
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.
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