I'm having an issue where the transmission will occasionally downshift by itself during a fast acceleration. I have a 2005 Suzuki Vstrom 650 and it's been a great bike so far. At pretty high revs, 9k-10k or so, it will occasionally go from 2nd -> Neutral, and I think the same thing happened once going 6th -> 5th gear.
What is causing this to happen and how can I fix it?
I've had this on a few bikes and it can happen for a few reasons. If it it's only happened a couple of times i wouldn't be too worried.
I've had a 'false neutral' when changing gears and instead of engaging the next gear the gears don't quite engage and you end up between them. Older bikes with higher mileage might be more prone to this. To avoid it make sure you're giving the lever good positive pressure and moving it the full distance of lever travel. It it also seems to help the gears engage if you hold pressure on the gear lever a little longer as you release the clutch. And maybe be a bit more gentle on the throttle when changing.
I have had bikes jump out of gear as you describe on a couple of occasions. I put it down to not having the gear properly engaged.
I had 1 bike that had a badly worn gearbox. 2nd gear was so badly worn that it would jump teeth. The edges of the gear dog teeth were rounded. I had the gearbox stripped down and had the gears 'undercut'. This puts a fine angle on the mating surfaces of gear teeth which means that under load they positively engage. This is not a simple task and should only be entrusted to an experienced specialist.
If it becomes a regular occurrence for you, you might be looking at a gearbox rebuild. Im not a vstrom expert but its possible there might be some adjustment or tolerances that might make a difference. Maybe even play in worn bearings might be a factor.
But as I said at the start if it's only happened a few times i wouldn't be too concerned.
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.
Cause:
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).
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.
Additional points:
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.
