78 GL1000 lost 3rd cylinder... but has spark and fuel

Hi all,

I'm a long-time lurker and seldom poster (I'm internet-shy) who generally searches through the archives for other people's solutions to the same problem I'm having.
I'm stuck enough to come out of my hole now, though.

I picked up a running 78 GL1000 for super cheap, and have been going through it. When I bought it, it wasn't firing on 1. I changed the plug, checked for spark, checked the float bowl for fuel, sea foamed her tank and crankcase, a few other things that escape me right now, and then discovered that the coils for two of the cylinders were swapped to opposite sides. When I changed them and put them back in the boots I fired her up, and the 1 cylinder started up right with the rest of em! I couldn't believe that was the problem - I couldn't figure out how it could have happened either. The PO seemed mildly knowledgable, but that kind of mistake would take some sort of intention or something.

Anyway, I've been riding around a bit repairing this and that as I figure out what's wrong, and yesterday when I opened her up a bit on the on-ramp it seemed to stutter a bit in the low rpms. After that I checked the cylinders with a spray bottle at the headers and found cylinder 3 was cold. I had thought I heard a little rattle in 3 earlier around 4000 rpms. I forgot to mention, I did the valves too. I can't figure out for the life of me what happened to cylinder 3. It was getting to the point where it was running pretty dang well before, all things considered. I have not cleaned the carbs, nor have a checked the cylinders for compression; because I'm a bit fearful of the carburetor and I do not know how to check compression, respectively. I think I need to buy some kind of tool...?

Please advise. I'm really excited to be excited about getting this bike on the road. I took 2 years off motorcycles for health reasons, and have missed it like you wouldn't (or likely, would) imagine.

Thank you!