Hi,

I got my first bike, a '91 GS 500 E of which I am very pleased.

Ok, so... when I am accelerating while in gear, I can hear some metal taps which become more and more pronounced as I firmly open the throttle some more.

I had this problem when I bought the bike, I did 100 km on it without doing a thing to it, and I took some diesel and brushed the chain, after which I used a rag to get the chain dry. After that I used a spray that acts as an anti-rust and which also lubricates the chain, not a high end spray expensive spray from famous brands, but something more modest.

I did not heat up the chain before or after the cleanup or lubrication and I didn't move the bike for a week after the operation described above.

Today when I rode the bike I could hear the metal tap is more pronounced and I am quite worried.

This is a video I did today with the bike in first gear, and you can clearly see the chain hops: http://www.vimeo.com/3790166
And this is a picture of the rear sprocket : 



My questions are:

1. Will repeating the cleaning operation described above do anything to fix the problem?
2. Is the chain too loose?
3. Is there something I did wrong in the cleaning operation?
4. I am quite certain it will need to be replaced, but can it last at least 50 more km?

Thanks.

in the video it looks to me like you have some major tight spots in the chain.  Tight links that is.  I dont know that i would try to get another 50 kms out of it.  Poor things was super neglected by the po by the looks of it anyways. 

That is one badly neglected chain!  Even after cleaning it's still rusty as hell!  Get rid of that POS chain and get a quality O-ring chain.  It shouldn't cost more than $70 and your problem will be gone.  If the problem is ignored the chain will soon break or fall off, and that's BAD for your bike, and your health!  Also, inspect the sprockets for wear and replace them if needed.  From the pic the rear sprocket still looks decent to me.

Also, you didn't say what you are actually using to lube the chain.  Use spray lube for chains or motor oil.  One is as good as the other, and lube it every two tanks of gas so it never gets in the condition the POS you have is in now!  Don't get imaginative and use anything that is water soluble or detergent based.  That means no D&L hand cleaner or KY jelly!  Use that on your girlfriend!   

Having said that, it may be possible that your chain is only rusty, but not damaged.  You can soak it in a coffee can full of hot oil until the links all pivot as they should.  You would have to check them one by one by hand.  Are you willing to do that, or just sh't can it and buy a good quality O-ring chain for $65 or $70?  I've never seen an O-ring chain so rusted that the links are stuck like that.  My guess is that it's one of those $35 non-O-ring chains that suck and shouldn't even be legal to sell for motorcycles.  A $65 O-ring chain lubed regularly will last for years on your GS 500.  cheap crapola chains

By the way, welcome to the board, and go over that bike to make sure everything else is up to snuff.  It's a good, reliable bike and we'll be glad to help.  Here's an online shop manual so you can read how to do everything you may need to do:

http://www.scribd.com/doc/7178815/Suzuki-GS500-Full-Service-Manual