right hand side of tank there's a little sensor with a breather tube coming from it (its you're manifold absolute pressure sensor) . when i had my misfire i went through everything from crank sensor,throttle position tests to ht tests everything u name it ... in then end i found if i pinched or took off and covered the hole it cured my miss fire ... replaced the sensor with a second hand couple of quid one and boom it was fixed when ever u held the throttle in a certain position or above it would miss like it was damp ... but pull her back and she would still power through it

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Yamaha xt660x 2009 blue/gold mods= iridium plug, stage1+2, removed snorkel, blocked ais, akrapovric pipes, power commander pc3usb (pre made map), adjusted tps, dynojet 02 optimiser 30amp aux relay, smoked puig screen, oxford heated grips and brembo carbon ceramic pads, rfx 30mm bar raisers, puig long adjustable leavers, plus much more and a cheep �30 alarm which stopped it been stolen