Topic: AHA!!! rebuilt cb400 carbs, idles high, slow to return to idle

[cmorgan47]

well, the subject says it all.  i rebuilt the carbs on my cb400f this weekend and over all it's much improved....more power, much more even power, no more gas leaks.  haven't really started adjusting them yet, and i've only been able to bench sync--don't have the guages, but will be borowing them from a friend this week.

the problem that i'm having is that, even with the idle screw about to fall out, the engine revs a little high.  the lowest it will idle is about 2000 and it is slow to return to idle from cruising.  choking it will cause it to fall immediately.

couple of questions:
  any ideas, or more info needed?
  should the throttle slides touch the botom when the throttle is closed, or should there be a small (1mm or so) gap
  maybe the trottle cable is a little tight?

[Gordon]

  any ideas, or more info needed?
  should the throttle slides touch the botom when the throttle is closed, or should there be a small (1mm or so) gap
  maybe the trottle cable is a little tight?

1)  Carb synch
2)  Very, very small gap, but not something that really needs to be measured
3)  Possible cause for the high idle, but probably not the cause of the slow returning to idle

[Bodi]

What did "rebuild" mean?
If you installed a complete rebuild kit - jets, needles, float valves, gaskets - make sure the float heights are set correctly (21mm) when you disassemble the carbs to put back the original jets and needles. Clean the slow jet with carb cleaner and a Q-tip, try and get some thread through the orifice and rub it back and forth a few times to really get any gunk out of there, then clean out attached emulsion tube cross-holes with some copper wire strands.
While the carbs are apart make sure the main jet emulsion tubes are clean too, if you skipped them last time. On dirty carbs they don't usually fall out and they are easy to overlook.
If the old airscrew tips look undamaged, polish them lightly with brasso or solvol and use them too. The tip profile is critical to the flow vs turns ratio and kit screws rarely match factory ones.
Kit parts have caused many problems for me. Unless the original parts are damaged or really worn (jets take a LOT of use to wear from fuel going through them) they should be retained.

[cmorgan47]

What did "rebuild" mean?

completely stripped down and cleaned(carb dip, pipe cleaners, compressed air and guitar strings)
new needles, jets, floats valves, gaskests....the $20/carb rebuild kits.

all is quite clean and running well other than the idle and the tech manual, lists un synched throttle slides as a possible cause for "slot return to idle."  looks like a sync is the first thing to try, so i'll be calling my buddy with the guages.

[cmorgan47]

so i had a eureka moment in the parking lot here.

apparently the lock nuts for the slide adjusters on carbs 1&2 had worked themselves loose.  tightened them back down, and it's idling much lower--imagine that... still runs rough, but havent synced yet.  will probably do that tonight, then put a drop of locktight on the nuts.