All RB mgb’s were made with what’s called a ballasted ignition coil system. This means that the car normally runs with a 6v supply to a 6v coil ….. achieved by a resistor (called the ballast) reducing the 12v battery supply down to 6v at the coil.
……. BUT whilst the engine is being started there is a direct 12v supplied to the coil to give the spark a boost during the starting process ….. which also compensates for the drop in voltage whilst the engine is being cranked over.
The problem is that you cannot see the ballast resistor!! The ballast resistor for the MGB is a short length of resistance wire built into the wiring loom denoted by the zigzag line in the WLG (white / light green) wire (shown in the wiring diagram) which supplies the 6v to the coil. The second WLG wire is a direct link to the starter motor and is only activated when starting the engine which supplies 12v to the coil ….. only whilst engine is being started.
Over the years previous owners may have decided to replace the coil for whatever reason with a 12v coil without realising the difference – there are no obvious markings to say whether the coil is 6 or 12 volts. If this is the case the coil will only be producing a weak spark due to only receiving half the voltage it needs. Also previous owners may have bypassed the resistance wire for some reason and fitted a 12v coil.
So you need to check if you have a ballast resistor wire fitted …… and you also need to check what coil you have to be able to ensure you have a compatible system
HOW TO CHECK IF A BALLAST RESISTOR IS FITTED
To check the voltage supply to the coil is not simply taking a reading on the +ve side of the coil as this will give a 12v reading if the points in the distributor are open – you will also get a false reading if the engine is running.
Remove all wires to the coil apart from the WLG wires and connect the other terminal to earth. Then with the ingnition on take a reading on the terminal with the WLG wires.
If the reading is around 12v or note then you have a 12v system and should be using a 12v coil. If you have a 6 or 7v reading then you have a 6v system.
HOW TO CHECK WHICH COIL YOU HAVE
If the voltage reading above is much above 8v you may have a 12v coil fitted to a 6v supply! So you next need to check the resistance across the coil. A 6v coil will have a resistance about 1.5ohms whereas a 12v coil will have a resistance of about 3.0ohms. Alternatively the coil may have a label to identify whether it is a 6 or 12v coil.
My car came with a 12v coil fitted so i thought when i replaced the coil for a ballasted coil and with a seperate external ballast resistor that would be ok ….. but that was not the case. The inline resistor wire was still in place so the coil was only receiving half the voltage so was only producing a reduced spark for me and the previous owner until i did the above checks!!
MGB tips mods and maintenance 
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