The Tohatsu or Mercury 3.3HP engine has been the most popular engine on the SB20. Thanks to a ban on 2 strokes these engines are always in demand for their lightweight and reliability.
While there are problems with them in general they have proven to be very robust nd parts are still readily available.
There a few common problems which you should consider when performing an annual service
Fuel System
The Carb is very basic and is gernally very reliable. I would recommend an annual strip down and clean. In my expereince the most common fault is gunk in the fuel bowl under the carb and the float not moving freely. What tends to happen is the fuel continues to flow in and then floods the engine. Carb can be taken off quite easily by removing the plastic cover ( Large screws on each side of the cover), cover splits in two and then comes off.
Then unscrew the two little screws which hold the black knobs onto the accelerator and choke lever. Then between the two levers and under the decal for the face plate are two screws which bolt the front plate to the carb. remove these screws and plate should come off. Then carb is removed by undoing fuel line and nut and bolt clamping carb to air intake.
The following link shows how the carb can be taken apart
https://www.youtube.com/watch?v=hFT6H2BYZAQ
Carb kits are available still from Cambridge Outboard Motors Ltd https://www.ebay.co.uk/itm/263924909067
Replacement is simply the reverse process. It is also recommended to replace the spark plug at the same time. The Spark plug is a NGK BPR6HS-10
Gear Lever
Another common problem is the gear lever seizing. This can result in it proving difficult to get into gear or not possible ot shift into gear at all. It is worth taking the gear lever assembly apart on an annual basis and cleaning, checking orings and regreasing.
The problem is that the plastic gear lever has two orings which are meant to stop water getting into a cavity where the lever attaches to the gear selection rod (which comes from bottom of the gearbox). The orings can let in some salt water and then seize up the lever.
It is a simple process to remove the access cover to the inside of the gear lever. Carefully remove this cover. look for the screw which then attaches the plastic lever to the selection rod. Once this screw is removed it is a question of careful easing the gear lever out of its housing. If this is seized then it will take a little careful working with oil and a lever or follow the procedure in the attached video.
Video of how to remove the gear lever (apologies not in English but still easy to follow)
New lever can be purchased from Cambridge Outboards motors Ltd.
https://www.ebay.co.uk/itm/282762513235