If it idles good the pilot jet and air screw combination are in the ball park unless the slide has to be open 1/8 inches to get the idle speed you want. 

If the slide has to be open about 1/8 inches to make it idle, the pilot jet and or the needle jet is too rich.  The slide being open 1/8 inches will make the choke not work and the pilot jet cannot work properly at cranking speeds.  Set the idle stop screw to provide the absolute lowest idle RPM.  A fast idle setting will make a bike hard to start.

How much compression does the engine have?

Make sure your reed petals are not not missing a piece or ****.  Check your pickup air gap on the ignition stator.  I set the air gap to .025" to .030".

Starting a 500 single cylinder is about 60 % technique, 20 % carburetor setup and 20% determining whether the engine needs choking or it may need the throttle open about 1/4 throttle because it is hot and partially loaded up.   ALWAYS position the piston at top dead center and the kick starter at the position that will rotate the engine the maximum number of revolution on one kick.  Kick starting with the piston up against the compression stroke on any 500 will make starting very difficult.

If you have the choke on, the throttle must be completely CLOSED or the choke will not work.