Congratulations on the new purchase....welcome to the site....There's a new rule around here...if you dont post pics within 2 weeks you're out!
Cant help you on the fuel economy but cant you pull the throttle handle out towards the center of the boat to disengage the prop? Thats the way my 87 mercruiser worked but maybe they didnt do it that way in 79?
Anyway...good luck with the boat