In this paper we present a on-field study for evaluating a crowd sourcing mobile social search application. With the help of the local crowd via social medias, this application assists foreign visitors in Japan by answering their image-based questions at hand in a timely fashion. We ran a controlled field experiment for 6 weeks with 55 participants. We found that the mobile crowd sourcing model demonstrated a reliable performance on response speed and response quantity: half of the requests were answered within 10 minutes, 75% of requests were answered within 30 minutes, and on average every request had 4.2 answers. Especially in the afternoon, evening and night, nearly 88% requests were answered in average approximately 10 minutes, with more than 4 answers per request. In terms of participation motivation, we found the top active crowd workers were more driven by intrinsic motivations rather than any of the extrinsic incentives (gamification incentives and social incentives) we designed.