Yamcha does pretty well here (despite his comic relief status). Cap is slightly over peak human right? Rogers is NOT at a supersonic speed level and cannot react to that level either (at least not that I remember). Yamcha, IIRC, helped beat the Ginyu Force when they arrived at King Kai's planet. I would imagine that puts Yamcha at supersonic/hypersonic (whichever) fighting speed, so Cap would most likely be getting speed blitzed.
Captain America is my favorite hero and I know he really doesn't stand a chance here. Yamcha has super speed, super strength, and has the ki force to at least destroy a mountain at this point. I'd go out on a limb and said Cap is the strait up better fighter but that isn't enough to close the power cap. The only way I really see Cap winning is if Yamcha thinks he is dead and starts to gloat(like the Saibaman incident) and Cap gets up and knocks him out with his shield. Of course that involves Cap surviving the initial assault, and with morals off that's pretty slim.