Won't start after standing still for two weeks

[JJK]

Today my gs refused to start for the second time.
The last time I rode was 18 december.

What happened:

Turn on ignition, choke is on, press ignite button.
Engine turns over but does not start.
Try again for some time.
After a while, the engine turns over slower, and some tries later does not turn over at all.
Also the oil pressure light goes off.
After waiting a while, the light comes on again and the engine turns over again after pressing the ignition.

Things I checked:

- Enough fuel
- Spark plugs are new

As mentioned above this is the second day that it refused to start.
The conditions that were the same for both days:

- Hadn't ridden for about a week.
- Cold outside (4 degrees celsius)

The previous time, the bike started fine in the afternoon.
The weather barely got warmer though.

Normally, the bike starts fine.

Any ideas?

EDIT: Starting it with the battery of our car does work, so I'm assuming a bad battery. I'm going to test it tomorrow. I do find it strange that it usually does start though, if the battery is bad.

Ps. I have trouble answering the verification questions:
How many kill switches does the GS have?
How many petcocks does a newer GS have?
How many petcocks does an older GS have?

[mr72]

Probably you need to put the petcock on PRI for a minute before starting after it sits. Then once it's running switch back to ON.

Definitely get the battery checked but it doesn't sound to me like the main problem. If it turns the starter, it should start. A battery tender is a good idea if you are going to be leaving it for a month at a time.

[JJK]

Thanks! Good suggestion. In the meantime I have replaced the battery, my local service center told me that it was definitely a diminished battery.

[peteGS]

During sevicing have you checked the valve clearances at all? Any idea how long since they were checked if not? On these GS motors hard starting is a symptom of valve clearances being too low preventing the correct air/fuel mixture being drawn in to get started.

Could also be simple things such as spark plug gap as well, I had that many years ago on my 450 before I knew better and the plug gap was way too big, pig to start on a cold morning.