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Honda BF25 High Idle

G

Green Oasis

New member
Hi All.
I'm new to the Forum,
We have Honda BF25 year 2000. After we bought the engine it would occasionally drop down from 3 to 2 cylinders. I removed the carbs, cleaned and replaced all the gaskets and seals. Afterwards the engine was running great. Over the past little while it takes longer for the engine idle speed to drop back down to the correct speed after warming up. Yesterday it refused to idle back down and although I adjusted the fast idle screw it made no difference. Other then the Idle speed the engine runs perfect.
Any help would be great.

Thanks Dan
 
Try opening the mixture screws up to half a turn more, idle mixture may be too lean, otherwise check for an air leak.
 
Hi Dan,
Welcome.
I haven't worked on the 25 so I'm certainly no expert with this outboard. It is a unique part of the Honda line up and probably has some very specific "quirks" that the unfamiliar, such as myself, wouldn't be aware of.

I don't know how important it is to synchronize the two carbs on the 25 but, judging by the emphasis on carb sync for the 3 and 4 carb models, it could be. Did you synchronize the carbs after doing your cleaning?

Is this a tiller control or binnacle (remote) control setup? If it's a binnacle, you could simply have a sticky throttle cable. The control cables really do need to be free of resistance to movement for reliable operation. When they wear they will hang up and, often, replacement is the only solution.

I see that the 25 has a dashpot valve. If that hangs up internally due to dirt or wear it could cause this symptom.

Have you looked at engine vacuum? It should be 8 to 9 in.Hg. and steady. Low vacuum can cause high idle. Although, this does sound like something may be hanging the linkage in this particular case. It doesn't hurt to take a look at everything though.

My final thought, for now, is about intake manifold leaks. Did you use new gaskets when working on the carbs? If you suspect any intake leaks, you can often easily find them by misting water (distilled) at all mating surfaces. An intake leak will pull the mist in and change the "tempo" of the engine speed for a brief moment.

Just some thoughts on your problem and I hope others here will chime in with their ideas. Most here are more than willing to try and help and back and forth discussion is always welcome.

Good luck.
 
jgmo said:
Hi Dan,
Welcome.
I haven't worked on the 25 so I'm certainly no expert with this outboard. It is a unique part of the Honda line up and probably has some very specific "quirks" that the unfamiliar, such as myself, wouldn't be aware of.

I don't know how important it is to synchronize the two carbs on the 25 but, judging by the emphasis on carb sync for the 3 and 4 carb models, it could be. Did you synchronize the carbs after doing your cleaning?

Is this a tiller control or binnacle (remote) control setup? If it's a binnacle, you could simply have a sticky throttle cable. The control cables really do need to be free of resistance to movement for reliable operation. When they wear they will hang up and, often, replacement is the only solution.

I see that the 25 has a dashpot valve. If that hangs up internally due to dirt or wear it could cause this symptom.

Have you looked at engine vacuum? It should be 8 to 9 in.Hg. and steady. Low vacuum can cause high idle. Although, this does sound like something may be hanging the linkage in this particular case. It doesn't hurt to take a look at everything though.

My final thought, for now, is about intake manifold leaks. Did you use new gaskets when working on the carbs? If you suspect any intake leaks, you can often easily find them by misting water (distilled) at all mating surfaces. An intake leak will pull the mist in and change the "tempo" of the engine speed for a brief moment.

Just some thoughts on your problem and I hope others here will chime in with their ideas. Most here are more than willing to try and help and back and forth discussion is always welcome.

Good luck.
Click to expand...

Hi jgmo.
OMG what a detailed synopsis. I had synchronized the carbs and all seemed ok. She is on remote controls and when in neutral position the throttle is fully off. Is the dashpot valve a non-return valve and can I test it?
I did replace all the gaskets with new ones so they should be ok. I will T piece in a vacuum and check vacuum

Once again many thanks for all the advice.
 
Ok....good info. Thanks for the feedback.

The dashpot is an anti-stall device, as you may already know, to keep the engine rpms "up" when the throttle is quickly returned to idle. See item 41 in the parts page link below.

You should be able to test it but I don't know the specifics for doing so on that model. I checked my Honda carb manual but that system isn't covered in there. The info may be listed in the proper shop manual but I don't have that one.

It does appear though that it's the only component and is likely just a one way check valve with a slow bleed orifice.

https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/25hp/bf25ay-sha-2000/carburetor

I will see if I may have the directions on how that valve is used on a cd I have but if you have the Honda shop manual you should look there first.

Good luck.
 
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