<p> KZ650 / Z650 Spark Plug Conditions</p><p>This information was contained in a free hand out published by NGK Spark Plugs back in 1977. </p><p>The appearance of the firing end of a spark plug graphically reflects the condition of then engine, the suitability of the spark plug heat range, the carburettor jetting and the ignition timing adjustment.</p><p>These pictures are intended to assist you in correctly choosing your sparkplugs and determining the performance condition of your engine.</p><p>Even plugs that present a good appearance, such as shown in fig 6 to 24, can quiet often be covered with lead deposits which cause misfiring.</p><p> Wet plugs (fig 1, 2) are usually attributed to one of the following causes o Excessive choking (wet with gasoline) o Trouble with the ignition o Oil pumping past worn piston rings and valve guides (wet with oil)  Dry sooty plugs (fig 3, 4, 5) are usually the result of o a plug with too high a heat range and the plug end not reaching its self- cleaning temperature (above 400C ~ 450C) due to light load o Too rich air/fuel mixture (richer that 8:1 ~ 10:1) o Trouble with the ignition system o Improper cooling system (if fitted) causing excessive cooling  Overheated plug ends (fig 25, 26, 27) may result from o Too low a heat rating permitting the plug to exceed the highest limit of optimum operating temperature (over 850C ~ 1000C) due to excessively heavy load o Too lean an air / fuel mixture o Ignition timing too advanced o Cooling system, if fitted, not cooling properly  Excessive Overheating conditions (fig 28, 29) are due to intense knocking / pinging and pre-ignition following situations identical to those in fig 25, 26, 27 where increased temperature of the spark plug firing resulted in the plug end melt down and significant damage to the engine. </p><p>Oil / Fuel Fouled Oil / Fuel Fouled Carbon Fouled Too Cold </p><p>Too Cold Cold or Rich Cold or Rich Cold or Rich But OK But OK But OK </p><p>Good Good Good Good </p><p>Very Good The Best Best Best </p><p>Best Good Good Good </p><p>Kinda Hot Hot or Lean Hot or Lean Hot or Lean But OK But OK But OK </p><p>Too Hot or Lean Too Hot or Lean Too Hot or Lean Too Hot or Lean Pre-Ignition Range Pre-Ignition Range Pre-Ignition Range Pre-Ignition Range </p><p>Too Hot or Lean Pre-Ignition Range </p>