In the back of my mind though, I'd always though perhaps it could be the ignition coil. The symptoms were similar under rainy, wet conditions as when I went through a bad coil period shortly after I bought Brigitta, my '87 R80 Airhead Beemer. Brigitta's condition though had resulted in the inability to hold idle while in the rain and really poor running condition.
Add into this mix, the fact that Ural ignitions type I through V have known issues with heat from the engine and the coil, so that on warm days, it can affect engine performance. To fix that, other owners had drilled holes in the plastic cover which protects the ignition module and coil. As I'd experienced firsthand the effects of heat on the ignition module, I'd done the same:
These photos were from the sovietsteeds forum courtesy of jpanyon, this is not Natasha but a similar Ural
However, as I am sure you can see it coming, the holes now let in water during rain! So, the last few rainy days I'd experienced some rough idling and poor acceleration from Natasha on my work commutes. Not good. A small shield in front of the vented cover had yielded mixed results.
Today I finally had some time to do some troubleshooting. First I made sure, while the engine was running, to spray water around the carburator hose fittings to see if I had air leaks, there were none.
Next I liberally sprayed water from a spray bottle onto the bottom half of the vented front cover, above where the "silver hockey puck" or Type IV ignition module is mounted. No effect, still smoothly idling engine.
Then I sprayed water into the holes at the top right corner of the cover, bingo! The engine started running rough immediately and soon shut off. Aha!
I removed the horn and removed the plastic ignition module cover and tried the water spray again with the engine running. This is what I saw, note the electrical arcing!
A good pic of the coil, note the silver tab parallel to the securing screws for the spark plug cables
the large silver colored round cover is the "silver hockey puck" ignition module
Above photo from Bill Glaser's excellent online manual for Urals
What those metal tabs are for, one on each side of the coil, I have no idea. They're not used and after some more testing I decided to cover them with some heat shrink insulation. I ended up doing both tabs and all the wire connectors as well. Everything on the coil is nicely insulated from water now except where the spark plug cables plug into of course.
I started the engine back up after the insulation work, sprayed water at the coil from all angles and no arcing or negative effects on the engine's performance!
Success!? We shall see at my next ride in the rain on Natasha.