Small Engines (Lawn Mowers, etc.): RIDING LAWN MOWER, fuel air mixture, 2 stroke engines

Question
MY SISTER HAS A WHITE (NOT THE COLOR WHITE)RIDING LAWN MOWER.IT HAS A 42 INCH CUT AND IS PROBABLY 14 OR 15 HORSEPOWER WHICH HAS STARTED TO SMOKE BADLY. SHE SAYS IT DOES NOT USE ANY OIL. SHE SAYS ALSO THAT MOWING AT A HIGHER MOWING SPEED,THE MOWER WILL JUST QUIT,BUT IF SHE MOWS AT A SLOW SPEED IT WILL KEEP RUNNING. COULD IT BE THE CHOKE?

Answer
Harold

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 an overly 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. See the sections on carburetor adjustment and carburetor cleaning.

   * White or black smoke 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 F 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 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 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, right?) burning off as well as white smoke/steam from accumulated moisture. If you tip the mower on its side routinely (to clean out grass clippings, for example), oil may seep into the cylinder resulting in white/blue smoke at startup as well.

     White or blue smoke while running may be an indication of an excessively worn cylinder or 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). Or, you may be using the fuel mixture for your 2 stroke weed whacker by mistake!