BMW /2 Transmission kickstart binding

Why does the kickstart lever on your /2 BMW transmission bind at times?

Let's have a look inside and I will explain.

Your kickstart lever is attached to a shaft at the back of the transmission cover.  This attachment uses a "D" pin which acts as a wedge to secure it to the shaft.  It is a good system, but it does need attention if it gets even slightly loose.  But that is for another discussion.

So, where does that shaft go?  If we open up the back cover of the transmission, this is what we will see.  The kickstart gear is the segment gear with the large spring, seen at the right of the back cover.

Let's have a closer look at what it does when you depress the kickstart lever.  When the lever is at rest, the segment gear is parked out of the way, in the similar shaped recess seen in the case.  When you start to push down on the lever, the segment gear rotates towards the idle gear (the only other gear in the back cover).  The idle gear rotates on a pin/shaft that is held in place at the back cover, using an interference fit.  Something to consider when putting all of your weight on the lever.

Just to clarify what happens next.  When the back cover is installed, the idle gear meshes with the input shaft.  These two gears are constantly engaged.  The gear on the input shaft ratchets to insure that no motion is transferred to the kickstart system once the motor is running.

Here is a photo of the input shaft gear that engages the idle gear (small gear at the end of the shaft seen at the left and without a bearing):

In summary, when you engage the kickstart lever it meets the idle gear, which in turn rotates the input shaft.  That is connected to the clutch disc, so if the clutch is engaged, the motor is turned over.  That's how the kickstart system works.

Now that we know how it works, let's look at what happens when it does not work.

Each time the bike is kickstarted, the same first teeth of the segment gear come into contact with the idle gear.  In turn, these teeth see the most use and do wear out.  Once they wear enough, the gears will no longer mesh properly and the system will bind up.

Let's have a closer look at it.  Here is what a brand new BMW segment gear looks like.  Note that the first tooth is just a partial tooth and there is a small leaf spring below it.  That spring will make contact with the idle gear and rotate it slightly, should the alignment be such that it would have clashed tooth to tooth.  The more on this later.

Once this gear wears out, it will look more like this.  In this case the spring is still healthy, but that is not always the case.

Let's look at this in action.  

Here is a photo of what would occur if the teeth were about to clash, but on a healthy gear the spring comes to the rescue to re-align the idle gear teeth (rotating it).  Note that the spring is pushed down in this photo by the idle gear teeth.

Here is a video of what occurs when in healthy condition.

Here is a video of a worn segment gear binding with the idle gear.  Of course, I set this up to demonstrate what would have happened inside the transmission.

Now that you know what this is about, how can you prevent it from binding, how can you prevent it from becoming this worn and how can it be repaired?

If this condition exists on your motorcycle, I would recommend that you have it repaired properly.  Of course, I can do that for you if you wish.  Just contact me at www.descoatelier.com.  But until you have it repaired, you will have to slightly rotate the idle gear if you find the kickstart lever binding.  What this means is that you will have to put the bike in gear and move the rear wheel fore or aft.  This will turn the output shaft of the transmission, which will turn the input shaft of the transmission, if it is in gear.  Make sure that the kickstart lever is not engaged, or you will simply ratchet the input gear.

To prevent this wear from occurring, learn how to kickstart your bike gently, making sure that you engage the segment gear and idle gear teeth before you bear down on the lever to start the engine.  Do not just slam down on the lever.  Engage it first and then kick it like you mean it. Of course, making sure that the bike is well tuned will also help it start easily without abusing the gears.

Lastly, what does it take to repair it.  Simply put, we have to get in there and replace the segment gear.  Just like any machine, it takes a trained eye to also identify any other problems while there.  To get to this segment gear, one needs to open up the transmission.  To do this, one needs to remove the output flange which requires a special tool.  In most cases, getting this far into a transmission means that it would be wise to go through it and make sure that all is well.