Outboard engine DIY

[csearle]

Today (and in fact a couple of weeks ago too) my friend and I dismantled and rebuilt a small 2.5hp outboard motor. It is a Suzuki. We learnt so much about how it works.

So the engine is a normally aspirated petrol engine that drives a hole with splines in it. It thrives best on fuel with no ethanol in it.

The big protrusion going into the water has a splined drive shaft running down inside it (with a small loop as a guide in the middle, a push rod for drive/neutral, and a pipe to carry cooling water up into the engine. The thing is hollow and carries the exhaust and the used cooling water down to an opening underwater.

Bolted to the lower end of this protrusion is the bit with the propeller, water pump, gearbox (crown + pinion), and dog-clutch. All the time the engine is running the drive shaft rotates as does the crown via the pinion). These are immersed in oil held in place by gravity. When the gear-push-rod is pushed down the dog-clutch is disengaged from the cut-outs in the crown and so the propeller is free.

On top of this propeller assembly a small enclosure houses a rubbery star-like wheel around (and driven by) the drive shaft. This pumps water up the pipe to the engine. Amazingly the space with the gear oil and the water are not ultimately separated. It is like the overflow of the gear oil is the same place where the water is sucked in.

Anyway we replaced a couple of parts (the lower engine cover and the gearbox housing assembly) that I managed to damage when boating to Foulness in a 28kt wind the other week. All is running well again.

Chris