I don't think you can use the vacuum reading to accurately set the idle speed because there is a pretty wide range of acceptable vacuum levels. The service manual for the 1977 KZ650-B1 says the range is 19 - 24 cm Hg. The 1981 KZ650 service manual doesn't even give the acceptable range other than to say if the reading is less than 14 cm Hg there is a problem. Both manuals are consistent regarding the fact that there should be no more than a 2 cm Hg difference between properly synched carbs. One other note, you mentioned that you got your carbs synced up today, but in order to synchronize carbs the idle speed needs to be set to 1,000 rpm +/- 50 rpm, so if your idle was too high or too low the sync may not have been accurate.
While you're waiting for a new tach cable, if you have access to a dwell meter you can use that to get an accurate measure of your engine's rpm. I think every dwell meter has a tach function built in. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
