i have a mercury 25 outboard engine,it has 2 spark plugs.2 boatmen have looked at it,and are not certain?if it needs 2stroke oil.there is no model number anywhere to be seen,the engine is a pull start,and a ignition key start,one of the boatmen reckons i should put 2 stroke in the petrol to be on the safe side?i have done this,but i have noticed,it gives off a lot of blue smoke,even after its warmed up,have i done the right thing,do the plugs need cleaning?? help would be appreciated. regards

brian gill, January 2009

Check if it has two valves on the cylinder head (springs and canterlevers probably under a small cover/gasket which can be removed). If so then its definitely a four stroke engine else a two stroke. Other solution is to take a sample of the crack case oil and let it analysed by a lab for its properties. They will tell you if it is 4 stroke or 2 stroke oil. I would not run it until you get a confirmation to prevent damage.

Paul, April 2009