A few things to check since you say it got worst are the plug wires and coil connections to see if you see carbon tracking on the coil or cracks in the coil housing. Next I would look at the carb foe any sediment or water in the float bowl. It's very common to have a problem like this, assuming it is carbed.
Either drop the float bowl inspect it for sediment and clean it out or use the drain screw at the bottom between the cylinders. Crack it open a few turns and let some fuel run out the tube in front of the rear wheel.
If you changed the plugs and it got worse my first thought would be the plug wires, they break down from engine vibes over time and pulling on them when you changed the plugs only makes the problem worse. I run Taylor wires and change them and the plugs every 10,000 miles.