The mechanic says carb is correct .runs like a raped ape but drive it 20 miles and uses a 1/4 of a tank of gas. Before it got 10-20 mpg

 

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You said before it got 10 to 20 miles per gallon. Before what? What changed? If it's running that rich then the spark plugs should be very black and carbon looking. If your quarter tank for 20 miles estimate is coming from your gas gauge then I would recommend filling the tank and driving 50 miles and then refilling it and see what mpg you get.