C7
I have never worked on or owned a Mariner,but marine engine "basics" are the same.
Might consider buying a manual(Clymers or Haynes) at local marine store for the Mariner engines. These manuals can be Big help(a bible of sorts) in making simple adjustments,trouble shooting all the way to a rebuild.

Have you changed plugs,adjusted carb mixture,timing could be off. Old gas,fuel tank vent not open. Check fuel line fittings,leaks all the way up to carb.Last but not least the carb could be gummed up???

When you push key in to choke, it activates a solenoid that in turn closes a flap or butterfly on carb that richins fuel mixture for cold starts.You should hear choke solenoid click when you push key in to choke. When you choke it - the engine should kick/sputter.Release the choke,engine should start the next time you spin engine. Gotta be real careful not to flood engine. If you can trouble shoot and correct simple problems on these engines vs taking to a marine dealer you'll save a boat load of money.

Let us know what the outcome is.