Carburetors main jets come in a wide verity of sizes. The jet number represents the cubic centimeters of fuel a minute that will pass through the jet at a low pressure - a head of 1ft(30cm). The higher the displacement of the engine the larger the jet size to satisfy the fuel requirement.
A 190 jet will pass 190 cc/min or 9.5 litres/hr (2.5 gals/hr). If this jet is found in a dual carburetor, twin cylinder configuration (Stock Triumph T120 or T140) each cylinder would consume this amount of fuel. The total amount of fuel used by the engine would be twice that amount of fuel used per hour or 19 litres/hr (5 gals/hr). This would be your approximate gas consumption per hour if the throttle were held wide open for an hour.
Single carb configurations (Triumph TR6s) with twin cylinders and the same displacement require approximately twice the jet size or a 340 jet. Twice the jet size to satisfy the fuel requirement for each cylinder.
Changing your jet sizes directly affects your gas consumption. A change in one size will result in a 10cc/min, or 0.6litres/hr (0.1584 gal/hr) fuel consumption. If you have a twin carburetor configuration double this to determine the total change in gas consumption of the bike.
When tuning a carburetor the fuel consumption is not really an issue. What really matters is if the motor is overheating (running lean) or running too rich. For a discussion on this topic check in the BBC Technique Web Page: What you Spark Plugs are telling you.