Atlas Copco XAS67 won't idle

I've just got a second hand Atlas Copco XAS67, but when it starts it revs to full rpm and when I close the outlet valves it just keeps reving and won't idle. I'm not sure where all the air is going, it's a little bit wet around the oil cooler but not lots. I've had the pneumatic cylinder that regulates engine speed off, rebuilt it and tried it on an airline and it works fine.

Any help would be much appreciated.

Thanks, Richard