Hi, Just want to know whats the performance difference between the 813 and 990? I know they are basically the same carb, except accelerator pump fitted in 813. I want a carb to improve my low down pick up, not worried about top end and was thinking that the 813 would be better due to the pump? Plus, Why is the 990 more expensive than the 813? would of thought it would of been the other way round!!!