Why does my bike start only when the fuel petcock valve is in the reserve position, and not when its in ON position?
Reserve on some motorcycles bypasses the vacuum valve
When you are using the 'reserve' position on many older motorcycles you are bypassing the vacuum operated 'ON' position of the petcock. The vacuum operated valve is a safetly feature so that fuel will only flow into the carburetor when the engine is running and creating a vacuum. There is a vacuum line that will origninate on one of the manifolds and terminate at your petcock.
You may have a couple of issues.
Hole in the vacuum line that prevents the vacuum operated valve from functioning in the 'ON' position
Disconnected vacuum line
Hole in the vacuum bladder within the petcock
Clogged petcock tube that rises into the fuel tank (low probability).
Check the above items and validate that your vacuum line is attached and does not have any holes or cracks in it.
When you Run your motorcycle On Reserve setting over Long period of time impurities tend to get deposited on the "ON" setting.
This may prevent the bike from getting started initially, All you have to do is fill up till half tank or so and while running at around 40 or 50 kmph turn the valve to "ON" position. It should free the valve.
If the above step does not work then there might be a clog in the valve.