Fuel from the main jet enters the carb at the centre of the primary venture - if you look down the carb, you'll see a circle held in place by a wing to each side of the venture. If you happen to have the float chamber cover off the carb, try pouring some petrol into the float chamber to raise the level above normal, you should see fuel pouring into the carb venturi from where fuel from the main jet normally enters the venturi. Most workshop manuals have a diagram of the carb parts, names all the parts and explains how it works, plus fault tracing. If you don't have a workshop manual, there's a free one available for the 500 in the Downloads section in the blue bar at the top of the page.
The main jet, and fuel from it, isn't involved in starting the engine. Air speed through the carb at starter motor speed is too low to cause sufficient depression across the main jet outlet, so no fuel emanates from here.
When starting the engine, fuel is supplied from the starter enrichment device (the 'choke') if it's used, and the idle fuel hole in the carb sidewall (very small hole just below the throttle valve (butterfly valve) when it's closed.
Your problem sounds more like a blocked idle jet. This is removable without striping the carb - look for a brass plug with a screwdriver slot on the side of the carb. Remove this brass plug, this is the idle jet holder, pull the idle jet from the holder and clean both, blow through the holes, especially the tiny hole in the end of the idle jet, reassemble and refit to carb.
A quick way to check if a non-start is due to lack of fuel supply, is to spray some 'Engine Start', or Brake or Carb Cleaner or else dribble a teaspoon of petrol into the carb intake, then try to start the engine.
Many people encounter difficulty when trying to start their classic car after being laid-up over the winter - please remember that modern petrol can 'go-off' when left for a long period, sometimes just installing new petrol solves the problem and avoids unnecessary fault tracing/changing parts etc.
Al.
