Recently I recieved a phone call from one of my customers who has a Honda LAWN MOWER which started smoking, so I explained him where to look at. Starting to ask a simple question, what is the color of the smoke?
BLUE SMOKE = Oil, BLACK SMOKE= Unburned Fuel, WHITE SMOKE= Water
Furhter more I explained him that for the most part, smoking is just as bad for a small engine as it is for you. Excessive smoke from the engine may be an indication of problems with the carburetor, rings, or gasoline.
Black smoke is a symptom of a to rich fuel-air mixture. This could be caused by a choke that is partially closed, a faulty carburetor, or the need for a carburetor adjustment. Make sure the choke if fully open. Check the carburetor adjustment and maybe carburetor cleaning is needed
White or black smoke on your lawnmower may also result from yard debris, oil, or other contaminants on the exterior of the cylinder as the temperature after a few minutes of operation will reach several hundred degrees Fahrenheit (or Celcius) even with proper cooling. Stop the engine and let it cool for a few minutes. Then, check around the cylinder, cylinder head, and under the shroud for grass clippings, leaves, oil or other spills, dead rodents, etc.
2 stroke small engines will always produce some fine white/blue smoke since the lubricating oil in the fuel mixture is being burnt along with the gasoline. However, excessive white/blue smoke could indicate an incorrect ratio of gasoline to oil or a mixture which has been sitting around for a while – the more volatile gasoline evaporates leaving behind the oil. It could also be an indication of contaminated fuel.
4 stroke small engines should produce virtually no smoke while running. At first startup of the season, there may be a few seconds of white/blue smoke resulting from the oil squirted into the cylinder at the end of last season (you did the preventive maintenance, correct? ) burning off as well as white smoke/steam from accumulated moisture. If you tip the mower on its side routinely, for cleaning out grass clippings, for example, oil may seep into the cylinder resulting in white/blue smoke at startup as well.
White or blue smoke on your lawn mower while running may be an indication of an excessively worn cylinder or piston rings or a clogged or inoperative breather (the breather assures that there is always negative pressure in the crankcase – if not, oil can get forced up into the cylinder). Also becarefull not to use your 2 stroke fuel mixture by mistake!
So the customer I was talking about earlier ended up with worn piston rings and damaged cilinder on his 4 stoke Honda lawnmower and indeed the mower had a white/blue smoke.
Sources: picture www.thisoldhouse.com, www.answers.yahoo.com,