An open decompression valve allows the piston to move without causing high compression .. so it's really easy to pedal /crank the engine over. Some engines are very hard to crank and some people do not have the leg strength to pedal-start an engine without this compresion release.
So, first a decompression valve cable would be activated, opening the decompression valve in the head. Compression is now vented.
Pedals then easily turn the crankshaft and the piston quickly moves up and down, building up some _momentum_
Now the decompression valve cable would be suddenly released, the valve closes, engine compression is developed and the engine fires (and hopefully starts)
First check for spark. Remove the spark plug, reattach it's wire boot and clamp the plug's metal base to the engine fins, grounding the plug.. When cranking you should observe a blue spark at the plug's electrodes. If you see spark, ignition is ok. If you see no spark, first try a brand new spark plug.
After spark is tested and affirmed, spray a bit of Starter Fluid into the plug hole, reinstall the plug and attempt to start. If it starts and runs for a second, fuel delivery is the problem.. the carb is not allowing fuel into the engine.
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