Fuel pressure relief valve adjustment/ bypass valve
http://www.peachparts.com/shopforum/showthread.php?t=234609
I am thinking it is the above or it is a Lift/Fuel Supply Pump issue. Unfortunately it is not easy to connect a Gauge to check your Lift/Fuel Supply Pump Pressure.
If it was a Filter/s Issue alone you would be having trouble going fast.
I would recheck your Throttle Linkage making sure your IP Throttle Arm goes all the way to the limit stop. Watch the Linkages while someone else steps slowly on the Pedal and see how much play is in the Linkage before the IP Throttle Arm starts moving.
Check the Bell Crank on the Fire wall that is between the Pedal Linkage and the Linkage going to the Engine.