Electrical Issue
 

Ok, I am very new to this forum but need some help. I bought a 1973 XJ6 that wasn't running, but got it going. We cleaned the carbs and put in new fuel filter and such. Sync'd the carbs, adjusted the timing and everything was running great. We would run it for a bit, watching the gauges, then shut it off. We went through this a few times and had absolutely no problems. I was getting excited and started cleaning it up to drive. Well then it happened. It started but idled at about 500 for a second then died. We thought it was plugged fuel lines but after replacing the filter we checked the pumps. No power. We checked the fuses behind the panel and the right side set had no power either. Everything (for the most part) worked well before this happened, and now barely anything works. I started looking at some wires before I lost daylight and found the main positive cable was loose where it attaches to other wires, but tightening it didn't fix my problem. My only theory after scouring this forum is maybe ignition switch is bad. Are there any fuses or main relays that would kill the power to half of the car? My fuses which don't currently work are 5-10, and neither fuel pump gets power. ANY help or ideas would be great. Hopefully this hasn't been covered before as I'd hate to repeat someone's post. (It shouldn't have been, since I spent an hour or so trying to find answers)
Thanks in advance!
PS: The sooner the better, haha. My truck's registration is up today and I was hoping the Jag would become my new daily driver.

OK, out on a limb here.

Trace the battery +ve lead, it goes to a threaded post on the firewall, and that has a corresponded +ve post on the other side of the firewall, and a battery cable joining them together.

Also INSIDE the car is the other threaded end of that post, and that is where the internal power comes from, NOT a known issue as its INSIDE the car, but the 2 posts that are actually in the engine bay are notorious for reeking havoc with power losses.

Take the first nut off each post (battery disconnected obviously), and give the cables/washers/nuts a good clean, DONT LOOSE THE NUTS, they are a wierd thread, and then refit and ensure they are nice a firm, NOT over tight, coz they will strip. When you have the cables off, check that the back nut is tight, again, nice and firm, not stupid tight.

Do the same for the battery earth cable where it secures to the body.

Ignition switch electrical section issues will affect the whole car, not part of it.