The easiest solution might be to use a zener diode to lock down the maximum output voltage to 9VDC. You would only notice the top 1/8 of your throttle throw no longer has any effect. Alternatively you could use some high-wattage resistors to make a voltage divider which would again limit the highest output but this would let the full throw of the throttle still be responsive. You should be able to find plenty of images of either solution on google so you can see what is involved.