Take the left plug wire and put in the right side, and vice-versa. See if the problem changes sides. If it doesn't, it's a fuel or compression issue. If it does, it's an ignition issue.
Fuel/compression problems: blocked main jet, torn diaphragm, broken throttle cable....
Ignition: worn out spark plug caps, old spark plugs, broken coil, bad connections....
"I don't know what you do, but I know what I do, and I don't do that." --Uncle Doug, R.I.P.
"Without the possibility of death, adventure is not possible"--Reinhold Messner