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1998 BF40 random high rpm idle

J

jaddie

New member
This is an odd one, at least to a non-expert like me. My BF40 will randomly go into a high RPM idle that's high enough speed that I shouldn't be shifting. Of course, if I'm docking, I have to. So...yeah, bad. The physical idle adjustment doesn't change, and it's not initiated or correcty with any physical throttle setting. The remote worm-up lever dosen't change or affect the issue, and the throttle/shift lever doesn't move. It will run at high idle for a period of time, then go back to normal. The motor otherwise runs very well. Any ideas guys?
 
Hi jaddie,

Welcome to the Honda forum.

An odd one indeed!

I don't know. I haven't worked on one of these.

But looking at the parts pages I see this model has the solenoid type choke mechanism. Is that correct?

If so, that's where I would first look for a problem.

I'm guessing of course but I think that either the choke being activated by a wiring or switch issue OR by something loose or broken in the mechanism itself might cause the engine rpm to increase and decrease as you describe.

I don't know how the solenoid itself returns to "off" after deactivation but I suspect that it has a built in return spring. If that spring were to break or become disconnected, it could allow the solenoid piston to flop back and forth when de-energized.

Item 6 in the link below


Other items of interest that might allow the choke to partially activate un-commanded would be items 26, 27, 28, and 29 in the link below.


Hope you find it and good luck.
 
The choke was my first thought too. But it retracts fully, and only pulls in when the warm-up lever is fully advanced. Also, when the choke pulls in, the motor does not run faster, it slows. But yeah, that was the furst suspicion.
 
Okay....
Strike 1.

Are you sure it's running on all three cylinders all of the time? If it were running on two and one started firing and then dropped out again, that might cause this symptom.

There's an active thread from Mark010 for his later model 40A where he talks about it running on only one cylinder. I've seen many outboards run without a cylinder firing and it's surprising how smooth they idle. Here's a link to that:

Honda BF40A outboard hard to start and only 1 piston is firing.

Hi, I bought a pontoon with a Honda BF40A outboard last Oct. It started right up and purred like a kitten before I bought it. Now, this spring, after indoor storage, it is hard to start and runs rough when it does start. A former owner said it was a 2010, but the emissions sticker says it was...
www.marineengine.com

Other than that, timing comes to mind. If ignition or valve timing is changing that could cause a speed up/slow down situation.

You might want to have a look at the pulsar wiring and connector as well as the timing belt and belt tensioner.
They're all depicted in the parts diagram below;


Good luck.
 
Yup, hitting on all, all the time. Silky smooth regardless of RPM.

It would seem that if there was an issue with the timing belt, the problem wouldn't go away though. Sill worth a check.

Pulsar wiring is certainly worth a look.

Thanks for the ideas!
 
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