I have recently bought a 1978 z650 and it stops when the engine is cold. It usually runs for 1km or so. Then i have to drain the carburetors and it starts again. I have tried driving with full choke. 50% choke etc. But it still stops. Any suggestions what it could be?

 

I have the same bike, a '78 KZ650B. based on your post ("It usually runs for 1km or so. Then i have to drain the carburetors and it starts again.") it's either getting too much fuel or not enough. To me it sounds like its running really rich, but if it'll start and run (more on that in a bit) it's getting fuel, but you just can't hop on and ride, you have to let it warm up a bit, for 2 or 3 minutes at least.

The KZ650 has a starting procedure outlined in the manual, and this is the way I do it: Petcock on Prime, starter switch on Run, Full choke, pull in the clutch (won't start w/o it pulled in) and hit the starter button. The engine starts and will run/race up because of the full choke, so I usually adjust the choke lever to where the engine is running reliably, and wait a minute or two for it to warm up, then switch it down to the Run position. FWIW, it should idle around 1200 RPM.

If it dies after about 1km ( I assume you're in Europe/Canada somewhere...?) it's not getting enough fuel to run as the bowls empty, which could be the pilot jets, the jets themselves, or the fuel line, which could be kinked or too much of elbow to the carbs from the fuel tank to allow easy fuel flow. The fuel in the carbs is burned, just enough to get you 1km or so down the road and then it runs out of fuel and dies. You wait a few minutes, the fuel slowly refills in the bowls, and it starts again but then the problem repeats itself. Very annoying, yes.

Get a manual, pull the carbs and go through them making sure your float height is correct, all the passageway are clear, you have the correct jets and they are clear, etc and see what happens. I would also check your oil to make sure you're not getting fuel in your oil. The carb floats *Should shut off excess fuel flow but not if they are not adjusted right, and you don't want gas in your oil. And another thing, you're not running pod air filters instead of the stock air box, are you...? Those create more problems than they solve. But it sure sounds like your carbs are off.

Yes, taking out the carbs is a hassle but you'll get pretty good at it after doing it 5 or 10 times - ask me how I know... - but its vital for the bikes operation, obviously. Synching or tuning them after you get them back in is a big help too - I use a Carbtune II device available on Ebay and it's indispensable for making sure they run right. It's not cheap ( around $100) but its worth it and can be used on any bike with a carburetor.

This is also very handy to have to measure fuel levels in the carb - $20 but worth it: https://www.ebay.com/itm/Kawasaki-Ca...EAAOSw~9JceCbr

Hope it helps.

-John