Anytime a motor has been sitting without being "winterized" (even if you live in Florida), particularly a carb model, runs the risk of developing "cooties" in the fuel system.
The 115 four cylinder employs what is sometimes referred to as a 2+2 system. The top 2 cylinder provide power at idle/just off idle (upto about 1800'ish rpms) then the second two cylinders kick in (that's a rather simplified explanation, but close enough).
So, any fuel issues are amplified at idle since you are really running on "half a motor" to start with.
Since it does start you can try the "quick fix" - which won't hurt regardless.
I would - change the sparkplugs (they are cheap), get rid of any old gas and start with fresh stuff. Add some Seafoam (or any "tune up" type additive) according to the directions on the bottle. Then check/clean out the fuel filter for the motor.
If the carbs need to be pulled/cleaned and possibly rebuilt, the seafoam will not do that for you. But if it's just some "minor" cooties combined with old gas and plugs, it could fix you up.
If after running a gallon or two of your concoction the problem persists, the carbs must come off, be disassembled and cleaned - new gaskets are a must when they are pulled and if you pull them, since you can never rely on the details provided by the previous owner (people just don't sell things that are "perfect"), I would rebuild them.
This can be an expensive process at a shop (count on at least $350 to start). If you are even slightly mechanically inclined a Seloc's manual ($35) and a couple of carb kits at about 35 bucks each will walk you through the process for about 100 dollars.
