OK, so you’ve got an old bike with carbs that are gummed up with old gas. If the seller said honestly that the bike ran well when parked, this process may avoid you having to tear down the carbs right away.
Put the bike on it’s center stand or support it upright, then drain the carbs. Tighten the drain screws – carefully, don’t overdo it, they’re small screws! Remove the tank, if possible leaving the fuel line to the carbs attached. Now, get a funnel that will fit that line – I use the tip of an old fashioned oil can but you can also use a temporary fuel tank if you have one, like a lawnmower tank – great for testing bikes. The objective is to fill the carbs with SeaFoam. Most carbs take about 1/2 to 3/4 of a cup of SeaFoam.
Once filled up, let the bike sit for several days. This is a perfect opportunity to drain and flush your gas tank and service the petcock if necessary. Drain the oil, replace the oil filter if used, and add new oil. Check the ignition system and ensure that you have spark.
After a few days, drain the carbs as before, tightening the drain screws when you’re done. Reconnect your tank, add some fresh gas, and pray she’ll run. This process has saved me a lot of grief a few times!