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Outboard vibration

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Hello all.

I have a fairly old 9.9hp 4 stroke Yamaha outboard which I used for the first time last week.

It pushes the boat along nicely but at mid range revs it vibrates - not overly so but enough to get annoying after a while. On the 4mph stretches the motor is quiet and vibrationless but on the 5 & 6mph stretches it vibrates the whole stern - enough to make your hands tingle!

I'm fairly sure it's not the prop as the vibration is identical in neutral.

Has anyone had experience with this engine and can suggest where the problem lies?

Any help would be most appreciated.

Thanks in advance.



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Hi Mark, we have a Honda 15 twin, when we had a similar problem ie it ran rough at higher revs, ours was a cracked spark plug.ie a crack in the ceramic.

At higher revs, the HT voltage is higher, and it then jumped to earth via the crack, so no spark at the plug. 

Yours might not be, yours could be dirty plug leads, or plug cap etc but worth changing spark plugs anyway, keep an old good one as a spare.

Edited by Viking23
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