What would MacGyver do… with a broken mower?

A little while ago, our mower started smoking like a bus queue… of dinner ladies.  It began as a $15 broken mower from revolve and it came close to returning to whence it came.

Lots of smoke with these old B&S engines apparently means one of two things: catastrophic failure or the need to replace or clean out a cheap part.

Step 1: head off

So the bore looks good, not that the photo shows much.  That’s reassuring.  But there’s plenty of carbon around the valves and the plug looks horrendous:

It looks like some kind of Liberal carbon party.

So there’s lots of oil in the cylinder, but the bore is ok (piston slid really nicely too).  My suspicion, thanks to the awesome power of teh interwebs, falls to the breather / valve cover thingy.  So it’s been sent to the naughty corner, which in our house constitutes a jar full of metho:

So it can think about what it has done, and hopefully reform its wicked ways.

The principle at work here is that to keep oil from being pumped into the cylinder and interfering with the fuel/air burning, the crankcase is kept at a low pressure through the operation of this valve, the breather.  The copper coloured circle is held up to the front of the silvery box by a spring.  When the pressure in the crankcase exceeds the pressure exerted by the spring, the disc is pushed back and gasses escape past it, through a tube in the block and into the air cleaner side of the engine, where those crankcase gasses are burned in a sensible fashion.  Somehow the thing has become clogged with or full of oil or something and this process isn’t working properly.

Next step is to clean the hole that runs through the block, the little pipe and the air cleaner housing where it emerges.

I think it’s the small engine equivalent of the PCV valve – a neat history of which is probably here: http://en.wikipedia.org/wiki/PCV_valve#History


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