hot water heater - sliding collar

I have a 17 year old hot water heater. I had to replace the thermocouple. Upon re-assembly, there is a sliding collar that can be adjusted to control how much air is mixed with the gas and thus controls the strength of the flame.
2 questions: 1 - how do you know when the collar is in the optimal position ? do you adjust based on flame color or flame height ? 2 - since the repair, i have a mild natural gas odor (maybe a bit stronger than mild) that exists when the hot water heater burner is running. it can be smelled throughout a good part of the house - any thoughts ?
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mark b wrote:

1-flame color. 2-you might have accidentally loosened the flare fitting which connects the main burner supply tube to the gas control. That would only leak when firing. You could soap the connection or call the gas utility to find out. They may be helpful setting the air shutter correctly too.
Jim
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