Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Tecumseh HM 100. My plug insulator is black like it's running rich ,buti t's not a sooty deposit. It's slightly oily and has what appear to be bits of semi hard carbon peppered around the interior of the plug and insulator. The exaust is not smokey. Any idea what engine condition would cause the plug to look this way? Thanks RDB

I can't say exactly, but they are probably oil deposits. These charts might help:

http://www.ngksparkplugs.com/techinfo/spark_plugs/faq/faqread2.asp

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

John Davies
Spoakne WA

the first thing to do is be sure the engine has the proper spark plug, too hot of a plug will result in burning lean, too cold of a plug will result in burning rich. out of adjustment points, or air gap if its electronic ignition will cause a weak spark thus incomplete combustion resulting in what appears to be a rich burn also. after acertaining all the above are correct, check compression both dry and wet test. if comp. is higher with the wet test, as a rule indicates worn cylinder/rings. if theres no noticible change, usually indicates a burned valve or other compression problem.

also check to see if your oil smells like gas,.....if you have a fuel leak through the carb, into the engine it will thin the oil allowing it to flow past the rings & getting excess, thin oil into the combustion chamber leaving residue on the plug. be sure your using the proper weight oil also.