Carb adjustment

[RobAch]

On 1980 Xs400, after carb rebuild, the bike seems to start ok but is idling at ~5k, way to fast. The adjustment screw, if correct in the photo, isn’t even in contact with the throttle. How then does one adjust the idle lower? What should it be? 1200? Thx

 

[amc49]

Vacuum leak or something hanging cable or throttle linkage up. You have to have air to idle hang high like that.

 

[RobAch]

So I just realized that working on the starter button, the handle was twisted a bit pulling on the cable. Even after putting back into correct position, idle is still 3.5 -4K. Ill look for more vaccuum issues but thought the only vaccuum line was the one to the petcock?

 

[amc49]

You can have vacuum leak issues even with no vacuum lines at all. May not even be a leak, if the carbs are not synced one plate cracked too far open will stimulate the slide to bump up to hang the idle. With that idle speed screwed to that loose the carb butterflies MUST be 100% shut to kill the engine or you have issues.

 

[xschris]

Sync the carbs with a manometer. If the carbs are dirty or one of your diaphragms are bad you will have this issue. Make sure the air filters/boxes are attached to the carbs and the exhaust is leak free. Could also be the intake manifolds have leaks.

 

[amc49]

If you have a good ear you can even sync by sound, I used to do it on twins all day long. Using 2 drill bits of same size can work too, smaller is better and careful not to ding the throttle plates doing it. I do 4 cylinder carb banks like that, often VERY close to where they end up after a true sync.