Topic: Idles to high when engin is warm

[Guyrob]

Hi

I have a 1976 Honda 400 F.  The carbs were rebuilt this summer.  Bike whent OK for a few hundred miles but now, when the motor is warm (after 15 minutes riding) the idle will be too high, I guess around 2000 to 2500 RPM (tach cable broken).  And yes, I adjusted the idle screw to the lowest RPM.

Air box and all other 'air connections' (excuse my poor English) seen air tight.

What may be the problem?

[toytuff]

Welcome to the forum eh?

Did you spray some WD-40 around the carburetor boots while running? Could be leaking.

Also, check your throttle cables for binding and leave a little play as per the manual.

tt

[BobbyR]

Are you by any chance adjusting the idle speed when cold? It should always be adjusted at full temperature.

[Stev-o]

First off welcome!  +1 on what has been said above.

[mrbreeze]

Welcome!!!.........my bet is that you need a carb synch. Been there and done that a few times. All other tune up steps should be performed before the carb synch..........you will be amazed at the difference!!!

[strynboen]

min 550 k3 and my cbx..have the same problem..i have veldet a litten  short "skrevdriver."....not now the english word...on the adjust screv..to get a better grib..and can easy adjust rpm..have syncro and tryed all. kabels and return  springs... but it still takes prm at hot engine

[flybox1]

your engine-side carb boots will warm with engine temp, and soften.  recheck the clamps and tighten if needed once warmed up. 
Who did your carb rebuild?
it might be that a bench sync set your slides too high so that when your idle screw is turned all the way out, the slides don't drop enough to bring the idle down. 
time to check your bench sync.....turn your idle screw in 1 full turn and then drop your slides down on the chuck-end of a 1/8" drill bit.

if you haven't done recently, set your valves and points.

THEN, do your vacuum carb sync.