Instead of a restrictor plate you could create some sort of throttle stop that would never allow you to go over 25% throttle.
For most gas engines, lots of throttle at low RPMs is the most efficient. This is because of the pumping loss across the throttle restriction. Why would you want to limit your throttle stroke to 25%? I use 90% throttle all the time and my MPG has improved since I started doing it.
brelandt 
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