Small petrol engine

[Octavius]

I have a small petrol driven garden appliance and somethings gone wrong!  I know I can buy a Haynes manual or similar for petrol mowers, but before that, does anyone know a decent site where I can find out the basics of how a small petrol engine works, the components and basic fault finding?  I imagine that it is something like a 4HP engine, although I am not entirely certain.  Cheers for any info.

[Clanad]

Is it a four cycle or two cycle engine and which company is the manufacturer?

[Gnu]

I have a petrol mower that sometimes stops working because the spark plug is all gunked up with oil/carbon - I clean it off with meths and then it starts OK.  This may not help, but it could be worth a look.