I posted the problem with my 81 Honda back in April and it has only just rained once since then, so I took the bike out to see if the new wires and spark plugs and caps helped.
I did eliminate one possibility before the rain stopped - venting the fuel tank didn't correct the problem so it wasn't a vacuum forming in the fuel system.
This time I was starting out with a cold engine, full tank of gas, and it was really raining hard. I didn't go more than two blocks before the engine started running rough, applying choke caused it to smooth out and run normally, turning off the choke caused it to run rough again, that worked through several cycles. Eventually I let it get too rough and it died. I coasted for about 30 seconds engaged the engine and it fired right up. This seems to work about as well as stopping to rest - engine will always start after a short rest.
I should have tried riding at very slow speed to see if it would keep going - but didn't think of it when I had the chance.
No amount of spraying water over the engine, ignition etc.. will cause it to fail in the driveway. I can dump buckets of water on the ignition and engine without it dying. Even a mist of water from a wet road will cause it to die and rain causes it to die faster.
At this point I'm almost ready to give up on an ignition problem since all the ignition except the transistor switches and pick up coils have been replaced. And the likely hood of two independent electrical systems failing seems too extreme. All the electrical parts have been drenched with water and it runs if the bike is stationary.
My latest and greatest theory is carburetor icing. From what I read, the conditions are favorable - humid between 20-70F sometimes up to 80 F. The problem is more likely to occur when the engine is cooler. It is more likely to occur when backing off on the throttle.
Where it doesn't fit the theory is that using the choke should exacerbate the problem - it doesn't seem to, and while the engine is relatively cool and the problem is occurring it actually helps. When the engine is at full operating temperature the choke is less likely to help it.
I can't get the problem to occur when the bike is stationary, it has to be moving through airborne water in the form of rain or mist (we don't get enough fog here but I suspect a heavy fog might kill it too).
My engine is adjusted too lean according to the color of the spark plugs. With "normal" carburetor icing this shouldn't exacerbate the problem - according to what I read. I didn't change the jets but did put in a less restrictive exhaust when the old one had holes in it - the problem did occur with the old exhaust but with holes in it so I can't say for certain mixture isn't involved somehow.
Any one with more ideas on troubleshooting this problem? Can it be carb ice with richening the mixture making it better? All I read says it should get worse as the mixture is rich, since they assume that the airflow is too restricted. But as the mixture is made rich, I could see where there may be less evaporative cooling from the gas since the gas should evaporate slower with humidity and gas concentration rising.
Have gas formulations changed in some way that might account for it?
I used to be able to keep going in torrential 30 foot visibility without it missing a beat - now it won't go more than two blocks in rain.
Once at highway speed, especially when going fast it seems less susceptible but backing off the throttle then, in rain, will almost always kill the engine.