Please ignore my last post. I was told it was an injector recall by the guy I bought it from 3 weeks ago. Managed to call JT Yamaha of Swansea, and it was a full injector O ring replacement. Still, feels seriously smooth now, even at walking pace.

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XT660R 2008 - Yamaha Screen - Touratech Bash Plate - Touratech Exhaust Guards - Wezmoto Braided Brake Hoses - EBC HH Pads - Continental TKC 80 Tyres - Pro Taper Pillowtop Grips - Polisport Handguards - NGK Iridium Spark Plug - K&N Air Filter