If I were to start typing all of the possible things it could be and you read them all you would be too old to ride by the time you finished reading.
Firstly go to www.jba.bc.ca/Bushmans%20Carb%20Tuning.htmlBushmans Concentric
pages, and read it all TWICE taking particular notice of the jet chart showing which jet dose what where.
Next start your bike and adjust the idle air screw.
If you can turn it more than 1/2 turn in either direction without drastically changing the idle speed then you idle jet is blocked, follow Bushman's instruction on how to clean it.
600 series comes in 3 sizes. The next two numbers are the size of the carb in mm what are they ?
When you pull the carb apart to clean it there will be a number in the underside of the slide should be between 1 to 4 including 1/2's
Tell us and we will tell you what size jets and slide should be in there. And even then this is just the starting point but at least you will be in the right state.
Go and buy a complete new set including the needle and slide the whole lot is about $ 50.00 and they are items that should be replaced regularly in any case.
DO NOT BUY THEN OFF THE CHEAPEST eBay
SELLEr YOU CAN FIND.
Get them form a reliable retailer such as the ones that advertise on this list or as you are in OZ. Modac, Trojan, Eades ,Classic Style, Mike's Classic parts, Motorcicclio ( others on this list will give you a few more choices )
They should come in a blister pack or in a box or labeled bag if they are genuine quality parts.
Do you have an air filter attached, is the element new ? If not then buy a new one of them as well ( another $ 20.00 ).
The reason why it starts on emergency position is that every last milli amp that the bike can produce goes directly into the coil, producing a much bigger, higher energy spark packet.
Because this is AC directly from the alternator it will not occur at the exact correct time but it will be a lot bigger so if a plug is fouled or your fuel is bad or ( as inteneded originally ) your battery is flat, the bike will start and run to about 1/4 revs before the inaccurate timing becomes a problem.
The fact that it will only start in that position is meaning less in this context other than telling us that the engine has big tuneing problems