Windscreen washer fluid works even better than water. Raise the revs up to about 4000 and put a small (3mm) vacuum hose in the jug. The alcohol blows the carbon off the valves when it hits. We take the old Mercedes v8's with efi and pour an entire gallon into the throttle body at about 5000 revs. The pile of carbon under the tailpipe is impressive. They run like a new car afterwords.
Cheers,
Dan