If it's blue smoke, it's either worn out to the point of scrap, or (more likely) it's been tipped on it's side and the muffler is full of oil - in which case it will burn off (eventually - 10 minutes or so).
If the smoke is black, replace the carburettor diaphragm. There are no adjustments possible. Get the diaphragm kit (very cheap), then proceed as follows ;-
1) Remove the air filter (1 long screw)
2) Remove the entire tank/carb assy from the engine (2 bolts - one just forward of the fuel filler, the other on the front of the engine below the carburettor). As you pull the assembly off the inlet pipe, unhook the throttle link from the carburettor. You do not have to disturb the governor spring(s) - just don't damage them/it. Sometimes, the sealing O-ring stays on the inlet pipe - if it does, pull it off and re-seat it in the carb before re-fitting.
3) Separate the carb from the tank (5 screws). Between the 2 you will fing a gasket and a rubber diaphragm - both included in the repair kit.
4) Re-assemble with the new parts. Make sure the surface of the tank and the base of the carb are CLEAN.
5) When you have it back on the engine, check that the throttle link (the thing you unhooked when you took it off) can move FREELY - no binding at all. If this gets jammed, you will have a runaway engine.
6) If all OK, replace the air filter. Job done!
All of this assumes a recent (primer start) engine. If yours is an auto-choke type (butterfly valve visible when you remove the air flter) let me know. The procedure is similar, but a little more involved.
phil_saunders(a)bigfoot.com
Phil Saunders
September 2012