Worn primary chain

I have a KZ400 with the primary chain at the service limit (20mm slack). However the guides are like new and the primary gear and clutch tooths as well.

I am looking for a new chain (hard to find), but in the meantime I am using the bike. Can someone tell me how bad is riding with a worn chain? My biggest fear is that it's going to ruin the teeth on the gears, which would be more hard to find.

Worn primary won't damage the teeth unless your gunning it's so that the chain jumps the teeth. 
When it starts jumping under light acceleration is when it definately needs changing unless the engine is out already in which case do it now. 
Don't know about your model but on 650's it's a complete tear down as the crankshaft needs to come out. 

IIRC,  and it's been a loooong time since I wrenched on a 400, the primary chain is inside the R.H./clutch cover.  Easy access!

Mike

Run it just make sure carbs are syncronised good to keep noise down they run pretty much forever

I come from the bicycle world, where a stretched chain ends up accommodating the rest of the transmission to its new length (eating the gear teeth for it)

But I will listen to what you say, it seems that for the moment I will be able to use it a little more. At the moment the chain does not jump or do anything strange, it's just that when I went to check the gear lever spring I took the opportunity and checked the chain.

I never understood why out of sync carbs can make the primary drive noisier.

The new chain costs about 200$.

Think of the service limit as the safe amount a chain can stretch without causing any problems or accelerated wear. All chains stretch. This is no different to a bike chain, they also have service limits, which really is just a line drawn in the sand for how much stretch is acceptable before the wear will accelerate. A chain just under the service limit isn't that different to a chain just over - but you need to draw the line somewhere.

Out-of-sync carbs make the chain noisier because each cylinder will 'pulse' the chain. If the carbs are balanced, each pulse will be the same strength and the chain will stay around the same speed. If one pulse is larger than the other, the larger pulse will pull the chain just a little more, and then the following weaker pulse won't keep up, and the chain will slacken slightly. So with unsynced carbs, the chain will be slackening and tightening slightly all the time.

wrote:

Out-of-sync carbs make the chain noisier because each cylinder will 'pulse' the chain. If the carbs are balanced, each pulse will be the same strength and the chain will stay around the same speed. If one pulse is larger than the other, the larger pulse will pull the chain just a little more, and then the following weaker pulse won't keep up, and the chain will slacken slightly. So with unsynced carbs, the chain will be slackening and tightening slightly all the time.

That's a great explanation, now it's totally clear to me, thanks

I know everyone calls it "chain stretch", but just to be clear which may help when assessing any possible damage that using it might cause, chains virtually never "stretch". Yes, they get longer; this is caused by wear where the pins rub against the holes in the links. On a new chain, there's very little clearance between link and pin - a thou or so maybe? When this clearance increases, the pitch of the chain becomes longer and this in turn causes more rapid wear of the chain wheels (and the mating link faces). The links themselves do not exceed their elastic limit and take on a permanent set.

Ian