It sounds like for some reason your bike is running too rich.
Have you cleaned the air filter? Also check the little three hole air inlet under the rear of the left foot plate. Critters like to make their nests in there.
Like daveysaves said the best thing you can do is get an expansion chamber but therein lies the problem. The Proma factory is out of business and the stocks of new Proma Circuit piped has dried up. The Leo Vince factory was offering a basic copy of the Proma pipe but if I am correct they have sold out all their stock and will not be making any more.
You can graft on another pipe of some sort but you will have to get a donor stock muffler and cut off the manifold and weld it on to the new pipe as the Honda connection spigots up into the cylinder.