No, the check engine light does not come on because your gas is half way full. The check engine light will come on if there is a problem with the engine.
Not unless it is a carburated engine.
the engine timing needs to be adjusted
Oxygen, since that is the gas trees excrete anfter taking in carbon di-oxide.
If you mean service engine light, then take off the gas cap and put it back on correctly. The fuel system is pressurized, if the cap is not on tight the check engine light will come on. At least three clicks.
Typically this means that the carburetor needs to be cleaned. The ports in the carb are not getting enough gas through them because they are gummed up from old gas sitting in the carb.
the engine.....i am just taking a stab at it
Because your foot is on the gas and something is wrong with your engine.
check engine mounts
DASHPOT
change the battery
you are flooding the engine
sounds like you may have a clogged fuel filter
It could be watered down gas. It could be watered down gas. It could be watered down gas.
if it is carburated it needs a little choke adjustment
Possibly a worn engine and/or gearbox mounting or exhaust mounting
See related link.