I have a Briggs and stratton 17.5 that will not start unless I put my palm of my hand over the air intake.  As long as my hand is over it it will run, but as soon as remove my hand the engine dies. Can anyone explain this?

Your carberator needs extra fuel when cold.
 
Your choke may not be closing when activated.  If the engine will not run W/O your hand over the intake once started your carberator is very likely clogged, (the main jet specifically).
 
Your, by hand, basically making the mixture RICHER as the carb is NOT providing the fuel needed. 
 
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