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40 should be plenty for the 13. If you really want to upgrade to the 50, get your hands on an OEM service manual and parts manual. You need to know the exact differences between the 40 and 50 in that year. For me it did turn out that the Nissan 40 in my year (2005) was a detuned 50. After several discussions with Tohatsu and consulting the parts manual I confirmed the difference was the carbs, reed valve limiting plates, intake manifold, throttle cam, sparks plugs, and timing settings. Everything else was the same. I purchased the parts for job used and cleaned everything up and rebuilt the carbs and I was in business. See here: http://www.whalercentral.com/userphot...lbum_id=65