So I was looking to buy a used Kawasaki z250, 1 cylinder bike that has around 5k km under his belt. Everything went fine untill I discovered it didn't get its first service at 1000km. I know that beside inspections it should get an oil change due to engine break in. How bad is it? Should I stay away? Thanks
The break in period is very important for any motorcycle engine due to the impact it will have on the REST of it's lifetime. A bad break-in can lead to major wear ( which can be translated to minor damage ) of the engine and also don't forget that metal alloys won't heal themselves like living organisms do. The reason that most manufacturers (for both cars and motorcycles) suggest an early (1000km) oil change after the break in period (instead of the usual 10.000km interval) is EXACTLY because this first oil SHOULD be changed as early as possible. This "break in" oil contains much more than usual metal elements inside it due to the initial wear of ALL engine moving parts AND clutch disks that when new will need to be "ground" till totally flat. You cannot (at least without a lab) see this debris in the oil but you get an idea by checking this initial oil being as dirty as the oil used in an already broken in motor after 10.000km.
It is not that the engine is damaged and it will not work but it surely has gone under a lot of wear (i assume that at the moment it has the same initial oil used for 5k km). So if we assume that the wear of it operating on this "dirty" oil is 3x the normal wear, your bike now has 1000+3x4000=12k km which could prove a great negotiation factor.
Your actual problem is that the previous owner actually neglected something very important and this could prove to be an overall approach on how he treated the motorcycle...
I would stay away if I where you. Changing the oil after the 1000km break-in period is very important due to the metal particles in the oil. This has most likely caused a lot of wear for how young the bike is and could cause issues in the future.
It also says a lot about the owner of the bike. If someone is neglegent about something as imported as the first oil change, one could imagine the owner being neglegent about all other aspects of caring for the bike aswell.
Again, if I where you I would stay away and find a different bike.