So my lil 400 is running great and now that everything is working fine, it started to kill its cam chain tensioners.
It already broke it 2 times. The spring just snaps and the engine starts rattling heavily.
It is clearly of the age. Does anyone know where I can get a new spring? My Suzuki dealer told me that he can not order one. He could only order a cam chain tensioner as a unit for 100€, and that is kinda a lot for just a broken spring.
To keep the bike running I twisted the end and gave it some tension. The bike runs without rattling right now, but I think it should be no more than a temporary solution.

If that was me I would'nt even start that engine with a bodged cam chain tensioner! If the chain skips a tooth or two it will trash the engine!
Have you checked if the chain itself is worn out and the tensioner is running at full extension?
Has your bike kicked back while trying to start it? As that could bugger something up.
You can buy manual tensioners for the bigger Suzuki's that might fit.

you need an aftermarket one that you manually tension yourself.

APE do them and SRAD 600 fits the bandit 4.

^^^ I knew it! Just didn't remember who it was that made them. DON'T over tighten it though!

The cam chain is totally fine and it did not skip a tooth or two.
I have done timing on this bike several times. As soon as it skips one tooth it loses compression on cylinders 2,3 an 4 without jamming the valves into the pistons.
I also measured the length of the chain. It still is below service limit.
The SRAD600 cam chain tensioner looks completely different to me.
Can someone get the exact dimensions?

Seems like I am going to buy a manual cam chain tensioner, or would be a new geniune one for 100 quid be a better deal?

depends on how often you have to pay attention for manual one. i think that most of automatic tensioner tension loss at longer expansion is caused by the age. their average age is somewhere around 20 years. warm-up / cool-down cycles probably softened the spring material during this period. if tensioner can be "opened", then maybe try to use some kind of spacer to increase spring tension? if it is not I don't think that 100 quid is catastrophic price.