Keinin CRS smoothbore tuning basics

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When I was a kid, I used to look at spark plugs and think I knew what I was looking at. In our motorcycle club, there was a guy called Barry Smith who was a works rider for Derbi. He once gave a talk on reading spark plugs. If you are tuning your Commando using NGK 8 spark plugs, when you look right down inside the plug to where the porcelain meets the metal, there should be a 2mm wide black band on the porcelain. Spark plugs are designed so that they do not foul. This is achieved by regulating the heat loss from the plug. An NGK heat range number 8 plug in a commando engine gives a setting point which is safe, a number 7 plug is even safer. But if you are using a number 8 plug and want best performance, the black band might sometimes disappear. With methanol fuel, you have unlimited antiknock, but with petrol detonation can occur. I don't use plug readings to set anything but the main jets. If you think about how often and for how long you use wide open throttle when riding your bike, even on big race circuits - it is not that much. So a bit of over-rich does not matter.
 
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With most race circuits, there is usually not a straight long enough to get a decent plug reading to get the main jets right. And with a race bike, you cannot do it on public roads. With a road bike, if you rode the bike a lot using mid-throttle, you might get plug readings which mean something, but I doubt it. If you are too lean in the mid-range jetting, you should know it because the motor will cough as you open and close the throttle when you are changing gear. If the plugs are black - start again. The carbs are probably touching the frame.
 
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