I bought a 2005 Yamaha YBR125 privately few weeks ago, it didn’t tick over very well then but I thought it just needed a good service. It starts OK but after it’s warmed up and ready, if you throttle on and off the revs hang up at 3000rpm for a few seconds then slowly down drop down to a tickover around 1500rpm. If the throttle stop screw is turned out any more it cuts out instead of ticking over. The carb, jets and air filter have been cleaned out, pilot air screw adjusted to 1 1/2 turns out, throttle cable adjusted OK and throttle slide sits down after closing the throttle.
Roy Wilson, email
It sounds like you have an air leak. You need to do a good visual check for cracked intake rubber; maybe the airbox trunking is off or has a clip missing.
If there’s nothing obvious, the next step is to get the bike hot and misbehaving and then spray the intake area with WD40. Spray the whole area with plenty of WD and once the engine sound changes put the straw onto the WD can and the more focused spray should enable you to pinpoint the air leak as the change in engine note is from it revving on more as it sucks the WD into the motor...But, if you are in any doubt about spraying inflammable liquid around a hot/running engine, get another pair of eyes on it or book it in.