Your Mechanic's diag. does not make sense. If you had water in the gas, then replacing the parts he did should not have made it run, if fact if it would cut off because of the water, it should not have started at all. In addition, a water condition will result in white smoke, not black. If your convertor was plugged to the point of causing it to not run, (if that can happen) you would have noticed a great reduction in performance long before this, like it wouldn't want to get out of its own way.

I think I would check around for a Mechanic familiar with your vehicle

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I really gets fun when they all don't run

84 500SL
99 Jeep XJ
93 Jeep YJ
03 Custom Softail