Most common and easiest to fix is improper air/fuel mix. What color is the spark plug?
If whiteish then clean the carb, clear the jet, make sure the air cleaner is on. If all these check out THEN look for air leaks. Do the spray test. If OK then think seals. Clutch side seal almost always smokes heavily when bad. Ignition side seal doesn't but is way easier to access.
If blackish, check the air filter screen in the airbox (the airbox pries in half) to see if clogged or rusty.
Ignition can also cause sluggish pick up. Read up on setting and cleaning the points and timing.