Rich/Lean - there is an optimum ratio of fule to air taht an engine needs. A bigger engine, bigger mans it pumps more air, needs more fuel. So you moved the 350 carb to the 305 and you have a carb 'expecting' to feed an engine about 12% larger than what it is feeding.

Add to that, the camshaft - it controls when teh vlaves, open, for how long and how far. An engine is a system- every part has to be designed to to produce similar results...meaning, a big cam (high lift) is used when you have the intake, carb and exhaust and heads designed to flow the air the cam is allowing in to the engine. Put THAT cam in your engine and it may run, but not very well (as in 'right' or 'efficient')

Generally speaking, everything is a compromise... you can have horsepower or fuel economy, but not lots of both. you can make lots of horsepower, but the most is made at high RPMs, not good for street use. A cam can be made for low rpm torque (good for trucks, towing and low rpm use) but if you wnat to race a Vette, you'll lose if this is your cam choice, BUT you'll be able to pull a house off it's foundation.

larger engines have larger cams - more lift and or duration (time valves are open as mesured in degrees of rotation of the engine). So a stock 350 cam in a stock 305 will improve performance overall - hore HP, more torque. Possibly more MPG..that depends on many other factors. All else being equal, if you engine makes more HP, it takes more fuel to do that. BUT if you make your engine more efficient (more HP from same size motor) then you can, in theory, use less gas pedal to drive at teh same speed so you may see better gas mileage...

You moved a motor from one vehicle to another, granted both chevy's, but different years, engines, etc. You should double check that all the vac lines are hooked up right.

Jets...the little items in a carb that control how much fuel can flow. the larger the jet (the bigger the hole in it) the more fuel can flow. The carb came from a larger engine, so it is jetted to flow more fuel than you need now. Also, teh rear end ration and or trans rations may have been a factor in jetting from teh facotry too (federal rules on emmissions and MPG are in effect here)