about 20 years ago, the hot set-up for curing all engine ails was 
water injection...

Spearco made (and maybe still does, i'm out of the loop) a water 
injection kit that would inject a mist into the carb throat on 
demand, usually triggered by vacuum.  the water injection quantity 
and 'turn on' point were adjustable.  the claims by proponents of 
this were

1. higher mpg
2. carbon removal
3. cooler running, smoother running
4. reduced detonation

maybe this would help you air-cooleds, it seems like a perfect 
application.  yes, its dated technology but so are some VW engines.  

does anyone have experience with these set-ups?  are they still being 
marketed in speed shops??  i never did buy one of these setup but i 
think i kept the literature, i could post it if interested.

-dan