I have a 1980 Johnson Evinrude 115 HP 4 cyl, dual carb that runs well in the driveway with "earmuffs"

lgmc is correct and do what he said, also get a timing gun check your spark that way make sure all coils are working also check the spark advance, check with a book or the dealer i think the spark advance should be about 28 degrees at 5000 rpm..(that's off the top of my head ,so check first), when you do it- put the boat in the water , not on ear muffs. as for the gas consumption , they suck up alot of gas, check for leaks but pther than that they are older and not as efficient as the new ones....i have twin 1984, 185hp and i burn about 25 gals an hour at ful throttle and about 8 to 10 per hour at idle speed...good luck