MG Car Repair: mgbgt starting problem, ballast resistor, volt coil

Question
hi barrie, i just bought a 77gt with no mot, and it started up today after some
fuel and a charge of the battery...thing is it'll only run while im holding the
key fully to the right in the start positon and as soon as i let it spring back it
stops running. ive yet to clean the carbs and check the timing and such but
just wondered if you what this could be as it seems like like simple to resolve
that i cant get my head around at the moment, thanks, neil

Answer
Hi Neil.  By 1977 the MGB had a cold start coil fitted, with a ballast resistor wire inside the loom.  It is basically an 8 volt coil plus a 4 volt resistor.  When starting, there is a bypass wire that puts the full voltage of the battery onto the coil, bypassing the resistor.  Both these wires are white with a light green stripe and attach to the + terminal of the coil.  My guess is that one of these wires has either come off or has broken.  

It also could be that the previous owner has fitted the wrong coil.  The resistance of the HT terminal should be around 2 ohms, whereas a 12 volt non-ballasted coil is around 3 ohms.  Check it out and get back to me.