The phenomenon is called as hitting "False Neutral" while shifting.
Basically when the clutch is not able to engage the selected gear properly the drive shaft stays in the mode of stasis . i.e simulating the effect of the bike being in neutral. The only difference between a regular neutral and this one is that you don't see the neutral light on the dash.
First of all if its not a common occurrence then you don't need to worry since almost all of the bike are prone to this once the gearbox is old except the ones with the quick shifters(BMWs).
- Old Gearbox.
- Loose chain.
- No lubricant or oil in the system.
- Badly maintained gearbox or bad shifts performed on a regular basis.
- Having a full clutch setup( i.e you have to depress the lever to its maximum)
Having a full clutch though being comfortable usually is one of the main causes of hitting false neutrals.I would advise to change the set up to half clutch depress.
Fixing the problem:
Unfortunately there is no way to "Permanently Fix" a false neutral issue, it depends on the bike and the gearbox and all of the above reasons, However you can do the below things to reduce the occurrence of hitting a false neutral on your next trip.
- Tighten your chain( Also check if the chain,sprockets are in good condition if not replace them)
- Add fresh synthetic oil to the bike.
- Have a Half clutch setup( Meaning you need to depress only half of leaver to engage the clutch)
When shifting gears, keep the gear depressed with a constant force until the entire duration of the clutch engage. This is a extremely effective technique to avoid unexpected "false Neutral" occurances. For example when you shift from 1 to 2nd your feet will be under the gear leaver so instead of a nudge , after the nudge keep holding the leaver till the clutch is fully released.
The above technique mitigates "false neutrals" almost in all situations. (Trust me I personally do this)
Tip: If for some reason you hit a sudden false neutral keep in mind to ALWAYS UP SHIFT , DO NOT down shift since on hitting the False neutral your RPM will shoot up and if you down shift it will put heavy strain on the gearbox.