Your KZ650 would have come with 2 throttle cables. One cable pulls the slides open; the other pulls them shut. Assuming both cables are present and properly adjusted you can pull the slides totally shut against the throttle stop by turning the throttle grip clockwise. The most likely cause of the slow return to the idle rpm is a vacuum leak. Make sure the vacuum caps are tight on the carb holders, and do a test for carb holder vacuum leaks. Here's how -
Warm the engine up to normal operating temperature.
Temporarily set the idle as low as possible without stalling the engine. It should be down below 1000 rpm. The lower the better because when it is set low the carb slides are closed (or nearly closed) and this raises the vacuum level in the carb holders. The combination of the increased carb holder vacuum level and the very low idle amplifies the effect of the leak test.
Once the idle is set low, spray carb cleaner around each of the carb holders where they bolt to the cylinder head and where the carbs attach to them. If there are leaks, the vacuum in the carb holders will suck the carb cleaner in and this will affect the idle. It may cause the engine to stall. If the leaks are at the carb holder / cylinder head mating surface you need to replace the holders. If the leak is where the carbs attach to the holders you may be able to tighten the clamps enough to seal the leak. Some folks use starter fluid or other substances for the test, but carb cleaner works best as it doesn’t evaporate too quickly like ether and it leaves no residue on the engine like WD40.
If you do need new holders just bite the bullet and buy them. Trying to seal things up with some kind of sealant or goop will only lead to frustration as it will at best only last a short time.
Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
