i had a similar problem with my suzuki dirt bike but it sounds to me like you have a leak somewhere..im not too familiar with cafe race bikes or sport bikes of the where abouts of filling the gas is. but with me my tank had a leak and that white residue is actually gas that has eaten the paint a little. what my problem was is that there was a hair line fracture in the tank and it got through to the paint. i suggest you look further more into this and see if it is leaking anywhere else! 
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