Spark plugs are an important engine accessory. Perhaps itís because they do such an important job so well, yet are often taken for granted. This little fellow has character. For the alert, knowledgeable mechanic, Mr. Plug is ever willing to reveal its secrets pertaining to the health of the engineís fuel system, oil consumption, combustion chamber, and even the engine treatment given by the pilot. At the Textron Lycoming Service Hanger, we have come to lean heavily on Mr. Plugís ability to "tell a story." Actually, heís our ace troubleshooter.
To make it possible for Mr. Plug to do even a better job, we are listing some "doís and doníts." These tidbits are directed at both the mechanic and pilot.
The massive electrode type spark plugs are the least expensive to buy and do a fine job. The fine wire platinum plug is more expensive but gives longer life, is less prone to frosting over during cold starts, and appears to be less susceptible to lead fouling. The more expensive fine wire iridium plug has all the qualities of the platinum plug, plus the fact that the iridium material resists lead salts erosion to a much greater degree than platinum. This results in longer plug life. So ó make your choice.
DONíT - reuse spark plug gaskets.
DO - use the recommended torque when installing plugs.
DONíT - be a throttle jockey. For years we have been preaching that engines donít like sudden throttle movement. Well, the spark plugs donít like it either.
DO - after a successful flooded start, slowly apply high power to burn off harmful plug deposits.
DONíT - close throttle idle any engine. Fuel contains a lead scavenging agent that is effective only when the plug nose core temperature is 900o F or more. To attain this temperature, you need a minimum of 1200 RPM, (TIGO-541 is an exception). Besides, the engineís fuel system is slightly rich at closed throttle idle. This ends up with Mr. Plug having a sooty face.
DONíT - fly with worn or dirty air filters or holes in induction hoses and air boxes, for this is the fastest way of wearing out engines. Mr. Plug doesnít like it either. One of his worst enemies is silicon (a fancy name for dirt).
DONíT - you mechanics, attempt to clean lead deposits from plugs with an abrasive type cleaner (an excellent way to keep the plug manufacturer on overtime filling replacement orders). Use the vibrator type cleaner sold by the plug manufacturers. Then, sparingly use the air powered abrasive.
DO - properly lean your engine in flight as recommended by the Pilotís Operating Handbook, and Lycoming Service Instruction No. 1094. In addition to being helpful to the engine in many ways, it also helps the plugs run cleaner, more efficiently and longer.
DO - you mechanics, be a little more careful in gap setting of massive electrode plugs. The top and the bottom of the ground electrode should be parallel with the center electrode.
DONíT - reuse obviously worn plugs, regardless of how they bomb check. More than 50% of the ground electrode eroded away; the center electrode shaped like a football; the center core of the ground electrode badly dimpled? If the answer is yes, replace.
DO - use anti-seize compound when reinstalling plugs. Caution: only sparingly on the first three threads. Here is not a case of twice as much being twice as good.
DONíT - accept dirty and stained cigarettes; they may cause misfire.
DONíT - use any spark plug that has been dropped. One manufacturer says "If you drop it once, drop it twiceóthe second time in the trash barrel."
DONíT - reuse any plug with cracked porcelain, regardless of how it may have been working or how it bomb checked. It will cause serious preignition.
DONíT - shrug off oily spark plugs. New, topped, or majored engines with some oil in the plugs is normal because rings havenít seated. High-time engine with oily plugs means rings are wearing out. One oily plug with others dry, means a problem in the cylinder with the oily plug. (The bottom plugs are always first to tell the story).
DONíT - clean plugs with a powered wire wheel. This is known as "a slow death on a fast wheel."
DONíT - you mechanics, determine replacement spark plugs by referring to model number on old plug in the engine. The man ahead of you may have installed the wrong model. Use the manufacturerís chart on all plug replacements: also consult Lycoming Service Instruction No. 1042, "Factory Approved Spark Plugs."