I don't know if this'll help, but when I take off in the Spit I don't build up power, I just push the throttle forward and start moving (after warming up the engine for a while).
Also, have you checked that your rudder pedals are moving in the cockpit (or the rudder itself in external view) before you roll?
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Core 2 Duo E6850 @ 3GHz / Radeon HD 5870 / 8 GB RAM / Windows 7 64-bit / X52 Pro / Track-IR 4
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