There is a spark on a negative terminal when starting

[machetta]

Here is what it looks like on youtube.

I assume I need to check wiring? Or possibly a starter motor is faulty? I have a multimeter and an electrical diagram of the bike (GS500K6) but not sure where to start!

[Badot]

Spark will only be there with a bad connection. Disconnect it, clean the contact points up real good, then put it back on nice and tight.

[adidasguy]

Oh, yes.
Cruddy, dirty connections.
Clean all your connections. Hit the Radio Shack for some contact cleaner to spray on after you clean them to help prevent corrosion.
While you're at it, disconnect things one at a time and check for corrosion. Spray on some cleaner then reconnect.
Corrosion in one place can mean more corrosion on other connections.

[machetta]

Not sure if the problem was that simple to fix but it actually helped a bit! Now I can hear something is rotating inside the bike when I press the starter switch. And it gradually comes to a stop when I release the switch. I assume it is a starter motor. However the engine wont start at all. Usually I can feel the bike vibrating a bit when the engine starts. Not in this case - it is completely motionless. I checked the battery - 12.1 V.
Could it be a battery? I personally don't think the battery is to blame here. I have had problems with the battery in the past but the symptoms were different (the engine would usually vibrate or make noise trying to start).

[adidasguy]

12.1v is too low. A battery just sitting there should be over 13v.

Remember what sparks are: Electrons jumping from place to place through the air. They are mini-lightening. Sparks always mean bad connection.

Congratulations for having a volt meter. So many people find that a frightening, foreign tool.

See what happens with a better battery or jump it from a car battery (car NOT running). 13.4 to 14.4 is a great voltage range to get your bike started.

[machetta]

Unfortunately the problem is not with battery  What I did:

1) I charged the battery
2) I used my car's battery to start the bike

Here is the new video showing the problem.

[adidasguy]

Could it be this?

[machetta]

Thanks, I think that's exactly the problem I have! Now I understand that the complexity of the problem is probably beyond my abilities 
I will have to call Road Assistance and have the bike moved to a repair shop.