So with 570 Thumper kit - Full Port and +1mm Valves - 43mm bore Carb - Reground 807 Cam by Web to keep some of my low-mid - Full HMF System: I should have plenty of ponies to call on?Yeah around 1.5whp in the mid and top going from 39mm to 41mm. With no bottom loss. Going from 41mm to 43mm would net another 1-1.5whp up top but loses about .5-1whp down low. I think its a sacrifice id be willing to take on the xc motor. Its got alot of bottom end to waste but needs alot of help up top. In order to get 43mm to work right they need to do a few more mods, but can be made to work just as good as the 41mm.
I plan on doing a 42.5mm on my 505sx.