Hi Mark, so if i set a meter to ohms scale and put one lead on the yellow wire at the coil and the other lead onto the steel part of the coil should i get a reading of around 0.48 ohms if the coil is good ? cheers..

one lead to a good ground, the other to yellow.

So. I have a reading of 0.8 ohms does this mean the coil is bad and if so can it be repaired ? Cheers

I don't trust Yamaha specs enough to say that... especially on this model where the specs are not easy to find.  In my experience if you get a reading anywhere close to correct on a lighting coil, it's probably okay. 

Have you checked for AC voltage at the yellow/white wire at the connector near the stator with engine running?  With no load, voltage will likely be quite high.   

Also, I know you said you checked for continuity...  probably with a digital meter?  That can be misleading because there can be a connection that allows 0.00001 amps to flow and meter will show "continuity" but at under any real load the connection fails completely. 

Powered test lights with an incandescent bulb is a far better test for lighting issues as it puts some load on the wiring system.  When I was a heavy-duty diesel mechanic we had a lot of tractors and trailers with lighting issues...  the other mechanics would spend hours trying to fix the issue...  mostly because they would get out their fancy meter and decide the wiring and connections were fine. I would get out my homemade powered test light and usually found the problem very quickly.   (What they didn't know was when I was a kid hanging around the truck shop at the company my Dad worked at, an old-time mechanic showed me a lot of "tricks".  He told me not to waste money on a fancy meter to chase lighting issues and built me a test light that would load the circuit by soldering a couple of wires to an 1157 bulb....  and he showed me how to use it logically and methodically to identify and isolate the bad connection...  many times it was a "scotch lock"  quick splice tap that would show continuity but fail under actual load.)     

 

so just put a 6v bulb onto yellow and ground and it should light up ?what controls the voltage/amperage ? looks like a very basic current generator ....

Yes, you could do that.  Just don't rev it much or small bulb will likely blow.  12 volt bulb would work too. 

The voltage is regulated by carefully balancing the maximum output of the permanent magnet alternator against the electrical load.  Which is why it's critical to have a battery and all the bulbs working.    In 1976 they started using an AC voltage regulator that can be retrofitted to your bike. 

Or, if you have a regulator [single wire AC Reg] just hook it to the yellow then see how high you can go with bulb wattage. On my AT1 'magneto model' i have 12volt quarts halogen headlight bulb out of my Honda Quad.
No regulator or battery & lights up super bright basicaly direct from yellow.
I mistakenly gave it a rev after a long hibernation one year & the original [replaceable] 6volt bulbs went instant "poof"--gone.
So just sayin--what am i saying--umm, should be plenty of volts hiding there.