There are basically two reasons a carb might be preventing the engine from starting .. a fuel restriction, or too much fuel is being delivered.
Things like dirt or rust anywhere in the fuel pathway can restrict fuel.
Things like a too-high float level, or leaks due to dirt or damage on the fuel shut-off needle and seat can cause flooding.
A stock bike will start and run fine with the carb and jetting the factory provided if the system is clean and tight...
First thing is to diagnose why it won't start.
Get a new spark plug.
Spray some Starter Fluid into the carb's air intake.
If the bike starts there is a fuel restriction of some sort. The carb is not delivering fuel.
If the bike doesn't start check for spark. Remove the plug, clamp it's base to the engine's fins. Crank the engine over and look for spark at it's electrodes.
If you see spark the plug may be fouling due to flooding once it is installed and an attempt to start is made.
There are many reasons other than carburation for a bike not starting .. wiring problems, perhaps a blown bulb or broken horn.. Search this forum for Jawa-related starting problems. And read FREDs GUIDE for more things to check and how to diagnose starting problems.
There's a Jawa manual HERE break>
The main jet's size determines maximum fuel flow at top RPM and has no effect on a bike starting. idling or running below 3/4 throttle or so.
After some performance modifications are done to a bike and after the engine is capable of running at higher RPMS than when it was stock, main jetting is checked, just to be sure that the carb can still deliver as much fuel as the engine demands at this higher RPM.
On a stock bike in good tune there's no reason to modify jetting or anything else.. The engineers set it all up correctly at the factory.. just return things to as-new condition.