Could be many things. Assuming a petrol engine!
What colour is the spark plug?
Black/wet ... running very rich/flooding :
a) Check carb (in particular that the float floats and that float needle stops/starts fuel flow.
b) Clean/replace air filter.
Is there a spark at the plug?
If not, try another plug, or could be HT lead or coil.
Compression. Remove spark plug and cover the hole with your thumb. Crank the engine and check for compression.
Could also be points/timing. But this is getting more advanced!
Mark Pate
July 2008
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