KZ305 Fires up and dies immediately, and then no spark

This might be a common issue, but I'm not familiar with it. I don't know a lot about ignition systems, but trying to learn. I'm working on an 82 KZ305A2 (or A1, not sure). I got everything put back together, carbs rebuilt, engine partially rebuilt, new battery, air box modded, etc. I didn't have the carbs connected to the airbox, and I decided to test it out. It cranked up just fine, but with so much airflow it just ran wild, so I ended up making more mods to the airbox and got everything all sealed up. A few days later I go to start it, and it fires up on the first try but then dies within about 2 seconds. It did that one more time, and then nothing. The starter turns, and it tries, but it turns out there's no spark. I thought maybe it was a wiring problem or maybe bad coils (still might be I guess), so I stopped messing with it and came back the next day. Same thing. It fired up, died, and then no spark. That was two days ago. Tonight, after watching some videos and reading about motorcycle ignition I decided to charge the battery and then try it again, but it did exactly the same thing. It was raining a lot last week, and I didn't have the bike covered. Could that have something to do with it? Or, does it sound like bad coils or something? Also, I guess I should mention that I checked the contactors and other connections that could have collected water, but everything seemed to be pretty dry.

Starting and dying in 2 seconds is common. How exactly are you arriving at the assumption that there is no spark? How are you testing this?

Huh, they have point ignition these bikes, lucky you. This'll be fun

If you have a spare condenser then put that in and try it. Are you familiar at all with points ignition?

I used a screwdriver against the engine to test for spark. It's definitely not sparking. Unfortunately, I don't have a spare condenser. I put a new condenser and points on it several years ago, and I haven't ridden it at all since then, but I have had it running during that time. I'm familiar with how the contactor works, but not with how the condenser works. I set the points timing, but I need to adjust the dwell angle.

Dwell meters are great but you don't need one to get the bike working.
Have you replaced and set points before? Do you have a manual?

Oh, a 12 volt test light is one handy little device for troubleshooting this condition.

Yeah, I downloaded a manual online when I first got the bike about 9 years ago, and I've studied it thoroughly. I've adjusted the points several times throughout the years, and I'm pretty confident in what I'm doing, at least with that part. I'll get to it with the test light this weekend and see what I can find. I wouldn't be surprised if the rain settled somewhere that I can't find and is shorting something out.

Are you 100% sure you have no spark? You have mentioned the air box a lot and that you have made mods. Also that without the airbox it ran, albeit badly.
It sounds like you have sealed the air intake to the engine with your mods and it starts, then consumes the air in the box so can't continue running. Leaving it for a few days and the air box refills so when you try to start it, it will, but then consumes the air in the box again and then won't start.
I had exactly the same issue on a 900. The PO had put a solid lid to the airbox on it when it should have had a mesh lid. Consequently the engine ran out of air after a few seconds.

I'm pretty sure there's no spark at all. I checked both sides, and I did it the same way I always have. The airbox mods are pretty simple. I basically just replaced the original rubber connections between the carbs and the airbox with silicone hoses to pvc adapters. and added another pvc adapter to connect the filter. When trying to start it, I didn't have the filter on, so there should have been plenty of airflow. I'm going to find my multimeter and test the resistance on the plugs, coils, condenser, etc. and check continuity on all of the connections. I'm pretty sure either the condenser has died or something is shorting out.

I just realized something. Water has been getting into the contact breaker, so I'm going to check and make sure there's no corrosion on the points.

That would do it. Make sure they are clean and not pitted.