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Old 09-13-2018, 04:36 PM
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Default Honda BF40 misfire

Hi, Can anyone please give me some advise about a misfire on a Honda BF40D carburretor model engine
The engine starts easily and will run fine at tick over speed OK without misfiring.
If I run it for few minutes and then try to accelerate it will badly misfire (on all cylinders I think) and eventually cut out.

If I turn the ignition key off for a split second , then restart it straight away and then accelerate it very quickly to maximum speed it will run OK at full speed without misfiring or any lack of power .

So the boat will either run at tickover speed (about 3mph) or at full speed (about 30mph) but not at any speed in between without badly misfiring. .

Could this fault be down to faulty CDI unit ?

I thought the fault may be a fuel problem
I have cleaned the three carburretors , cleaned out everything in the fuel lines , changed the fuel ,changed the tank and connecting fuel line
fitted new spark plugs. All with no effect .

The cylinder compressions all read between 145 -150 lbs

Any advise would be greatly appreciated,

Thanks , Pat
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Old 09-14-2018, 01:40 AM
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welcome to the forum have you checked the condition of your coils.
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Old 09-14-2018, 04:31 AM
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Thank you for your welcome to the forum and your reply.

Although I think I could fit any component myself ,as I used to be an auto mechanic many years ago( before CDI units were invented.)
I do not have any way of checking the coils or the CDI unit as I do not have any knowledge of modern electronics.

Is it possible for the coils just to faulty when accelerating and then be OK at full speed and could all 3 go faulty at the same time ?

What ever the fault is, it is effecting all the engine and not just one cylinder.


If you think it could be an electronic problem with coils or the CDI and not a fuel problem it will probably mean me taking the boat to a main Honda outboard dealer to sort it or possibly buying an expensive CDI unit and trying it .


Many thanks, Pat

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