it should say in your manual what the correct float height is, before you go bending anything check to see if the floats are already at the right height, then we can eliminate this as being the problem.
make sure it is nice and warm before you start tinkering with the tuning side of things.
sounds to me like it is running rich when you open the throttle, my DT sometimes does that when i start it up first thing and have maybe left the fuel tap open, it clears itself after a few seconds though. if your floats are already at the right height then maybe try a smaller jet.
surely raising the needle would richen the mixture?