AKA Hot Start problem.
This has been bantered around on other threads and it applies to all bikes with the (2009+) Chinese-made single cylinder engine produced prior to mid-June 2012 (engines numbered before 61712400). Supposedly, problems are occurring with 1 out of 30 of such models.
It refers to a situation where your bike is running fine, you make a short stop and then have trouble starting it. The bike will not fire-off quickly as normal but lets the starter grind on and on. The battery is so small that not many grinds will run the battery down and you're dead in the water.
Often, in this situation, even after the battery has worn-down, if you let it sit for about 15 mins, the engine will cool-down and the battery will recover enough to pop the bike back to life as normal.
Many affected riders simply think their battery is getting weak and they replace the battery. A new, hot battery will overcome the Hot Start issue for a while but eventually the symptoms return.
The problem is the "decompression lever" attached to the end of the exhaust cam. This item is designed to fall into place and hold an exhaust valve open slightly during the initial start procedure. This relieves cylinder pressure enough to give the poor little starter motor and tiny battery enough of a chance to spin the motor quickly enough to start the motor.
In some situations, the decompression lever is not opening the exhaust valve enough to provide enough decompression relief. The motor won't spin fast enough to start.
The problem is caused by either of two things. The the valve lash clearance of the affected exhaust valve is too great and thus the decompression lever does not get enough push on the valve to open it sufficiently. Also, temperature variations in the related componentry vary the expansion and size of the related parts to a degree that adds to this issue of not allowing enough opening of the decompression exhaust valve.
The fix-
A new decompression lever has been offered by BMW,
part 11 31 8 540 965 (about $50)
that replaces the original lever. Also, checking the valve lash clearances is recommended to make sure there is not too much slop in the valve train.
BMW issued a service bulletin on this in 2012 and it is covered by any current warranty. They agreed to cover this problem for bikes out of warranty until December 31, 2013. If you are outside that category, discuss this pre-existing, known-condition with your dealer.
https://www.dropbox.com/s/5ubnydbxjy2yl7e/hotstart.pdf
The decompression lever is part number 2 in this illustration-
An actual photo of the cam assembly shows the decompression lever pivoting on the end of the cam.
The decompression lever is weighted and falls into position to add an additional bump on the valve bucket. Once the cam starts sipping centrifugal force causes the decompression lever to swing out of position and allow the lobe on the cam to be the only force on the valve bucket.
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