If it is like similar Hondas I have had, the fuel air mix is too rich, and your throttle cable needs to be tightened up as well, so it will close all the way when you release it or rock it "throttle down". Full and complete combustion are not taking place and unburned fuel and air are getting into the exhaust system and being ignited...kaboom! You might check the exhaust valve gap and timing as well if adjusting the mix to a leaner mix does not completely fix the problem.
LEEDTex
