Oh and ps re rich and lean, simple version is this: all engines need a mixture of fuel and air to run. If you blocked off the air intake and tried to run on just fuel, the mixture would be too RICH and the engine would not run. If instead you tried to run on air then of course the mixture would be too LEAN and the engine would not run. So you need a correct fuel/air mixture for the engine to run correctly. That is also one reason why the after market filters, the Kev fuel mod and the after market exhausts are so useful as they all allow the engine to breathe better and thus to be more efficient.

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(Now sold on, sob) 2011 Blue XT660X with gold wheels, was 26,500 km.
Engine mods: K&N Stage 1 filter, DNA Stage 2 filter, snorkel removed, Kev fuel mod fitted.
Plus: smoked Puig screen, Yam aluminium sump guard, Yamaha / Acerbis handguards, Givi Trekker aluminium side cases, Leo Vince X3 cans plus Kev front fork mod and Fender Xtender. Just fitted Michelin Road Pilot 3 tyres.

SOLD the XTX for a Super Ten 1200. And just bought a Raptor 700 so staying on here for some tips....