Ignition circuit

 The original ignition circuit on the MGB was a very simple affair indeed and nearly anyone with a wiring diagram, a multimeter and some common sense can maintain and troubleshoot it without much concern. The MGB is a simple 12V negative earth (they originally started with positive earth as was the fashion at the time) circuit with the main 12V running directly from the batteries (or battery if you have gone back to 1) behind the seats in the cockpit to the starter motor under the distributor on the driver's side of the engine. This end of the thick 12V wire goes to several places to power the entire car including the fuse box, the ignition and headlight switches and the alternator. A white wire runs this power off the ignition switch, through the tachometer and onto the coil. This supplies 12V directly to the coil with a white/black wire running from the other side of the coil to the distributor. See, simple ...

Well, that part is simple and should be checked if you are having trouble with your ignition circuit but, there's a bit more to check if you're having issues. 

There is an old mechanics saying that 90% of carburetor problems are actually ignition problems meaning that before you go and blame those wonderful SU carbs and start winding screws and jets around to fix your issue, look at the ignition circuit. This section could be a whole blog in itself but there are plenty of sites out there to help you with these issues. I will say that if your car is missing under most circumstances, check your tacho. If the tacho is dropping suddenly when the misses occur, it's probably the low-tension side of the ignition which means the circuit I just described above. If the tacho is jumping around the misses, it's probably the high-tension side. Now like I said this section could be a whole blog but a jumping tacho could also mean your 50+ year old tacho is starting to give up as the original electronics (caps, resistors, transistor etc.) are drifting and dying. I will write a separate section on rebuilding your tacho in both original form and using an aftermarket PCB to bring it up to modern standards elsewhere. 

If you suspect the white wire running from the ignition switch, through the tacho and on to the coil, which is a switched wire providing 12V when the ignition is in the ON position, you can take a switched feed off the fuse box directly to the coil or you can take it from the multiplug in the engine bay with the other white wires running out of it. This is a switched 4 bullet plug connector which runs between the ignition switch and the fuse box and runs the fuel pump at the back of the car. It has one spare input which you can run to the coil without worrying about checking for power if your fuel pump is working properly. Your tacho won't work with these set ups but if the car runs you know it's that wire between the ignition switch and the coil.

If it is your high-tension side, then don't panic. There are still only a handful of parts in this section and the usual calm and methodical check and replace will track down the issue and probably cost you very little except a couple of parts and tea and biscuits. Read through the page I wrote on your first service as a start. With the high-tension side cleaned and checked see if the car now starts and runs better. If not it's time to start replacing small things first like the condenser (inside the distributor cap) and leads but always keep your old ones if swapping them doesn't make any difference. They're good for parts later if a new part fails. Also always check your engine earth strap which is usually on the front timing cover plate to the engine mount on the driver's side. The engine needs (and I mean needs) a good earth to run properly so even if you think this looks OK, it might be time to remove it, clean both ends and where they bolt to and refit it. To show you how serious I am about this I run 3 (yeah THREE) separate earth straps to my engine/gearbox, so I know if I get a miss, it's not an earth. While you're checking earths also check all the contacts on your batteries, both positive and earths.