Originally Posted by Maximo

Yeah the 9901/41021 is big time holding the engine back.... 9901 is a 3 chamber muffler pipe primarily designed to reduce noise to cater to some senseless and arbitrary European rules some stupid fools came up with about 10 years ago... the 41021 manifold doesn't help much either.... a swap to a 9853 or 9886 on a 41001 will wake that beast right up...especially if you add a bigger carb insert like a 8 mm

edit....even adding a 41001 to your 9901 will make a big difference...for some reason the 41021/9901 combo has terrible top end and just adding the 41001 will give the engine a huge boost