Usually is a bad electrical connection which, with water added, tend to short out a bit.  Check the spark plug wire and cap, and connection to the coil.  If you don't see anything amiss, try running it parked in neutral while hosing a small section at a time (eg top of motor, headlight, etc around carbs) until you make it stutter. Then at least you'll know the location of the problem.  Keep us posted please.

wrd17 wrote on 08/20/08 at 15:03:59:

I agree with Trippah, it sounds electrical to me.  One point of clarification though.  "it tends to run very rough" - is that just in general or is that only when wet?  I assume you mean that all of these symptoms are related to the rain and/or water but just want to be sure.  This is more of a clarification for those on the forum that understand such things because other than asking a few questions I doubt I can help fix the problem.  

I would start it & make sure its running nice enough, then start wetting it down, hose, spray bottle, something.. be creative, but remember, that plug wire is serious bidniss.

The squirt bottle does seem more refined a method,  I'd start at the battery (and that includes the ground for the battery)and then follow the juice up through to the spark plug (which) I guess would include the lump on the back fender and the connections under the gas tank (sorry)>   Anything after the spark plug shouldn't interfer, and I think the kill switch, sidestand cutoff safety stuff would make it a go/no go rather than sputtering etc so I'd save them for later..