Switch off

Hi all, I have a 1979 st70 6volt, I've been having a note mare with this bike, when it starts it runs perfect, but only starts every blue moon, I've changed every thing, twice,the only thing I can think of now, when the bike is switched off what happens to stop it, I'm thinking along the lines of what turns it off, is stopping it from starting

All switching it off with the key does is to disconnect the power line to the coil and that stops the spark. If it wont start then take the spark plug out and with it still connected to the HT lead hold it pressed up hard against the outside of the engine casing (anywhere metal), with the ignition key switched ON kick the bike over while watching the spark plug and you should see a weak spark jump across the gap in the spark plug (do it in the shade or somewhere dark). If you've got a spark then that's step 1 done. Then you need to check your contact breaker gap is set correctly as this determines the ignition timing which is critical to starting and smooth running. Once thats done then you need to check and adjust your fuel mixture on the carb.