I am assuming that by Japanese 3K you mean a Toyota 3K engine which is quite small if memory serves, 1100cc I think. I would guess that the carb is too small, or at least not any better. Anytime that you take a carb off one engine and put it on another odds are it won't be jetted right. If your going to do anything with the carb I would recomend going to Weber 32/36 DGV. If you want to stick with wrecking stuff I would try to get something off an engine that made more power than the 4AC (22R perhaps?). Getting jets for OE carbs may be problematic though, with the Weber you can get what ever you want.