Wednesday, September 15, 2010

Honda Foreman

2008 Honda Foreman TRX 500 FM

This machine is a great example of Honda engineering.  It is nothing fancy or high performance, but well thought out and durable.  The engine is a basic single cylinder four stroke motor with push rod actuated overhead valves.  It is air/oil cooled with an electric fan.

This is a standard ATV here in the village. These machines will last forever if you keep oil in them.  Unfortunately people seem to have a habit of running them out of oil.  The normal engine disaster starts just after someone changes the oil.  They either strip the threads on the drain plug and it falls out a few days later or they put the oil filter in backwards.

The stripped threads are definitely operator error, but the upside down oil filter is an engineering mistake by Honda.  The filter is a cartridge type with a hole on one end and a spring that pushes on the other end.  If it is installed backwards the hole for the oil to flow through is blocked off and the engine does not get any oil.   I have seen these problems many times.

The lack of oil typically leads to an overheated piston and cylinder.  If you run it long enough like this the piston breaks.  The rod and crank have roller bearings, so they normally survive fine.  Being a push rod motor, the cam is low in the block and it normally has enough oil to survive.

I'm not sure what is wrong with this particular one yet, it smokes pretty bad out the exhaust, uses a lot of oil, and sounds a little "rattly", but it still runs ok.   I'll pull the top off tomorrow and see what it looks like inside.


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