Gearbox jumps or slips out of gear
When accelerating or backing off, a gear 'jumps' out: the box slots itself into neutral on its own, the bike loses drive and the revs flare, sometimes with a violent jerk. It's a mechanical symptom not to be ignored: it often starts with one gear (frequently 2nd or 3rd) dropping out under load. Causes range from the simple — a wrongly adjusted shift lever or linkage, an incorrect clutch free play that prevents clean engagement — to the serious: worn, rounded dogs (the teeth that lock the gears together), or bent/worn selector forks that no longer push the gears fully home. Riding for a long time with a jumping box worsens the wear and can break teeth inside the case. Adjusting the shifter and clutch is within your reach; dogs and forks require opening the engine, so a professional.
Possible causes: gearbox jumps or slips out of gear
These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.
Wrongly adjusted shifter / linkage
BasicThe foot shifter acts, via an adjustable linkage on many models, on the selector drum. If the linkage is out of adjustment, the rod bent, or the shifter travel limited (a stop, a lever set too high/low), the gear isn't fully engaged and can pop back out under load. It's the first point to check, with no engine work: make sure the lever returns to centre, that the travel is complete, adjust the rod length and re-tighten the linkage bolts. A tired shifter return spring gives the same vagueness.
Worn gear dogs
ExpertInside the gearbox, the gears of a given ratio lock together via 'dogs': frontal teeth that mesh. Under jerks, missed shifts and mileage, these dogs round off and take on a bevelled shape that, under load, tends to eject them from each other: the gear jumps. It's the classic cause of a box that jumps out of a specific gear under acceleration. The repair means opening the engine and replacing the gears concerned (and often the forks too): a specialist workshop job.
Repair to be left to a professional.
Incorrect clutch free play
BasicIf the clutch doesn't fully disengage (too much free play at the lever, a badly adjusted cable, or on the contrary zero play causing slip), the gears are still turning relative to each other at the moment of the shift: the gear engages poorly and can jump. Setting the lever free play to the recommended value (often a few millimetres of play at the lever end) is free and fixes many imprecise shifts. Check it before blaming the box itself.
Bent/worn selector forks
ExpertThe selector forks guide the sliding gears on their shaft to engage each ratio. A bent fork (often after forced shifts or an over-rev) or one with worn pads no longer pushes the gear fully into mesh: the ratio stays on the edge of engagement and jumps under load. The symptoms overlap with worn dogs, and the two often go together. Diagnosis and repair require opening the cases: for the professional only.
Repair to be left to a professional.