Drake is correct! When working the 318, always run at full engine speed and modulate your ground speed with the hydro. If there is some problem with the hydro which will not let you regulate the speed then you need to be finding out why. If you are slipping tires are you sure they are mounted the right direction. Do you have pictures of your setup you can post here?You never work the tractor at idle. Full throttle only. You use the hydro control lever to limit your ground speed and keep the tires from spinning.
You also want to learn to use the split brakes.