Standard heat range Spark Plug (per Clymer manual): NGK DPR8EA-9; gap 0.7-0.8mm
On the 3 recommended spark plugs. One is labeled standard (as above), one labeled hot,
and one labeled cold. Which is the best choice and why?
"Heat" ranges do not mean ambient temperature. They relate to the temperature range the
spark plug tip experiences while the engine is running.
A "Cold" plug is manufactured so the electrode is just peeking out of the ceramic insulator.
This way, the spark plug tip is "cooled" quicker by heat transfer to the head. Racing engines, or engines running octane boosters
often use these.
A "normal" heat range is between "cold" and "hot". Normally there is more than one range
in the middle.
A "Hot" heat range has the electrode sticking farther out from the ceramic insulator.
This causes the electrode to run hot. A good use of this is an engine that burns
oil. You want the electrode to "burn" itself clean. A good use of this is a two-stroke, where you mix oil with gas. A bad
use of this heat range could be a hot spot in the cylinder. It could cause the fuel/air mixture to ignite BEFORE the spark,
causing pre-ignition. The worst use of this heat range could possibly get SO
HOT, to melt the dome of the piston a little each fire. The result is it burns a hole in the piston. Two strokes have this problem more than four, because the "philosophy" is usually different.
Pick the COOLEST plug you can run in a four stroke, and still do the job (plug light
Pick the HOTTEST plug you can run in a two stroke, and not damage the system (plug light brown)