Reviving an old thread, as I'm having similar issues with throttle pedal effort . . .
Last night, my buddy and I isolated the components (pedal, pedal spring, carb linkage, carb linkage springs). The excessive effort I have been feeling is definitely the throttle cable. I have checked all cable routing and the bends don't seem excessive between pedal and carb. The most severe bend is just as the cable leaves the pedal box, heading to the port side and then turning aft toward the cockpit.
We're going to try disconnecting the cable at both ends and working some dry graphite lube in there, perhaps with compressed air assist. I have used dry graphite on several speedo cables in the past, with very good results. I don't want to resort to wet lube/oil, unless it's the last resort.
The springs on each end (pedal and carb) are definitely NOT the primary cause of the stiffness. They contribute some, but aren't the culprit.
Any thoughts?