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Message: [QUOTE="jaguar, post: 1476083, member: 19340"] Look at the plug of two well tuned engines, one that is air cooled and one that is water cooled and you'll see the plug of the air cooled engine much darker which means it needs more fuel to keep from seizing due to the extra heat. That's one example. Also look at the plugs of a low compression and high compression engine to see the high compression engine's plug much darker which means it too is getting more fuel to keep from seizing. So jetting all boils down to hopefully getting the most power with the restriction of keeping it fueled enough to keep from seizing. Jetting is not about seeking out one color shade on the plug or a certain width of color band on the bottom of the plugs ceramic. For example I used to race a stock KDX200 with Pro Circuit pipe in 90-100 degree weather in Paraguay and once when I was changing main jets I found a size that gave the best power and then seized it. With that hot weather the jetting needed to be richer than optimal for power. Same thing goes for any of the other test methods. Knowing the piston heat in relation to the engine oil used is the only universal method I can think of for bringing the jetting to the ragged edge without seizing it. Here's my web page about this: [URL="http://www.dragonfly75.com/moto/plug.html"]How to Choose a Main Jet[/URL] [/QUOTE]