[chumbucket]

Must be all that new fresh gas you just gave it. ;D Seriously though, if it ran good when you did the sea trial and now it's not, I would start looking at the changes that were made first. While a tune up certainly wouldn't hurt, try to troubleshoot one thing at a time. Just double check all your hose connections, bulb, etc. to make sure everything is tight and not drawing air into the line. When it runs rough, was the bulb staying hard? If not, there could be a restriction (kinked line, or something in the line from the previous tank) or air being sucked in from a loose hose connector.
If everything checks out, then look at the tune up. If the fuel supply was as bad as you said it was, chances are, the previous owner tuned the motor with poor fuel and possible low gas flow to the carbs. He probably made some adjustments to the motor to compensate for the problems with the gas. If you think you're comfortable working on the carbs yourself, pick up a manual for the motor that you've got and follow the directions. It's not really a difficult job, but can sometimes be tricky. Otherwise, bite the bullet and pay a mechanic to work on it for ya.
I hear ya on straining the budget. Welcome to the world of boat ownership. It's always about straining the budget. :'(