MG Car Repair: MG Midget wiring, fuse box, column switch

Question
Just fixed my horn, added a second one and found that the reason it was not working was a bad contact at the fuse box. I cleaned the connections and horn works great. Headlight flashers work too, which is good because I had thought that the column switch might be a dud.
Took the car out for a spin and although the engine idled fine, it started to misfire and backfire when I accelerated. Made it home again by keeping the revs low. I wondered whether the horn was related to this and so I removed the fuse and when for another spin. Car ran fine, no problems.
Question: why? Could it be bad wiring, or could be something more serious.
My car is a 1968 mkIII 1275cc, with positive ground. However, all the wiring diagrams I have seen show this model to be negative ground. I was wondering if this was an issue - whether a previous owner had switched grounding for some reason. Note that my tacho doesn't work.

Answer
Hi Peter,
The horn circuit is a purple wire that has nothing to do with the ignition circuit unless it is shorted and drawing too much current which would normally have blown the fuse. I would check to see that the charge rate is correct (13.8 to 14.5v at 1500 RPM) Also check the coil connections to see that they match the battery ground. The points are a ground so the coil must match what ever is set up for a ground on the battery. If your car is set up as a positive ground then the + post of the coil MUST go the the distributor.

All 68 Midgets were negative ground so I can't see why anyone would convert it back to the old version of positive ground. You can't just switch the leads on the battery. The generator MUST be polarized to match the battery ground. That is probably why the tach don't work. Also you will probably find that the heater fan does not perform well because it is turning backwards.
If you want to change it to the negative ground you MUST read how to do that in my web site page about setting the polarity on an MG. Go to, http://mg-tri-jag.net and open "Tech tips" in the menu and read #1 tech tip.
Howard