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I'm verry limited by time so I have to find out quick. The cam chain tension seams verry week because the tensioner goes all the way up.
Is this normal? The bike runs fine but it makes a sound like the engines with weekened cam chain. I inspected the tensioner and it seams fine. The left chain guide seems to have that spring property. What could cause this problem? Could be the cam chain guides too wared?

It's a very small possibility that the guides are severely warn...most likely cause is that the chain has gone too slack and needs replacing, they last about 40,000 miles before they need replacing.

You will need to redo your valve clearances after replacing the chain.

Thank you banner! My bike has only 25.000 miles but... Do you know where can I buy one cheaper than € 31 (motorcyclespareparts.eu)? Why should I recheck the valve clearance if nothings change on cam shafts? I will do that for sure but I don't understand what is changing.

The new tight cam chain "pulls" the cams downwards than a loose one. You'll probably have slightly smaller clearances on cylinders 3 and 4 and maybe bigger on 1 and 2.
I'd also replace the rear chain guide as it mostly have signs of wear and sometimes breaks if it's too old.
40.000miles is in my opinion very much for a zxr 400, I have my third now with 50.000km. Have changed the last before necessary-> 40.000km head gasket blow. Business as usual lol

Can someone tell me what are the required sizes of allen keys needed for the screws indicated in the below images? I don't have my bike near by and I have to collect first the tools needed to change the cam chain.

5mm i think, but get 6mm just in case (im not near my bike either). you will need a 19mm spanner and socket to turn the timing rotor once its back together, and to assist you in removing the timing rotor.

you will need to remove the cams as well, thats a 10mm socket from memory

Yep, I know I need 10 mm to remove the engine top cover and the camshafts, 8 mm to remove the pickup coil cover and I think the key needed to rotate the crankshaft is actually 17 mm but I have anyway both 17 mm and 19 mm. I will take the 5 mm and 6 mm allen keys. Thank you!

Dominik wrote:The new tight cam chain "pulls" the cams downwards than a loose one.

What??? So the cams are located on some rubberised surface are they? FFS, what a load of rubbish. Valve clearances are measured with the cam lobe away from the valve stem, there is no way that the clearance can be in any way affected by a new cam chain / tensioner. However, as you have all the top end open you may as well do a valve clearance check to save buggering about a few miles later as a routine task,