Noticed on posts above that you may have had water in your tank at some time. If this is the case then it is more than likely that you have water in the bottom of your carb (as oil/petrol sits on top of water). Now, the feed pipe in a carb that supplies the fuel to your accelerator pump run from the bottom of the float chamber (where there is now water!) and so you will be adding an extra squirt of water just when you need a bit more fuel... Not ideal really! Take top of float chamber and have a look under float, you should notice if there is water under the fuel in there.
Mark.
I'm Not A Complete Idiot... Some Bits Are Missing!