Next, release all the pressure from your keg and keep the relief valve open even after the pressure is gone.
Then, unscrew the gray (gas in) coupler off of your CO2 line.
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It is important that you have the pressure out of the keg and the relief valve open when you hook the CO2 up to the keg, especially if you don't have a check valve in your CO2 line to prevent beer from being pushed back up and into your regulator.
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Next, hook up one of your black (beer out) couplers to the gas line instead (temporarily). Then, hook the co2 line up to the beer out post so that you can drive gas all the way down to the bottom through the dip tube. Set the pressure pretty low (like 3 or 4 psi) and keep the pressure relief valve open while you turn the gas on. Let it bubble like that for about 5 or 10 mins and you should be able to scrub a lot of the ass smell out.
Then, disconnect everything, switch back to the gray gas coupler, reset the pressure to serving pressure, and try it again.
HTH-
