Topic: Slow to return to idle

[Dinola]

I rebuilt all 4 carbs on this newly acquired 1976 550K that was barely running when i got it.  Used new full kits that were 38 slow/100 main.  Starts and seems to run strong but it was very rich (most signs said so) and on revving, it was very slow to return to idle.  When i blip the throttle hard, it just runs on and on at 4 or 5000 rpm.  I have to crank the idle knob down to below a sustainable idle to get it to come down, then raise the idle back up to keep it running.
Believing this was a very rich condition, I moved the needle clips from 4th groove to 2nd and finally to top groove where it now shows signs of being lean (won't always take throttle smoothly) but it still won't settle down after a high rev.  I worked the air screws from 1/4 out to 3+ turns out,, well,, where they just about fall out.  Seem happiest at 2 turns out but it still keeps on running at high RPM when revved.  I balanced with a mercury I've used on other bikes. Checked for air leaks, rechecked points and used a timing light, new air filter.  Hey, it runs great, it just won't settle to idle after a rev. I see the throttle linkage return to the idle position. 
During the rebuild I was very careful to set the float heights at 22mm and there is no sign of over flow. Using the choke at any time kills the engine.
Shees.... can someone guide me to a solution?  I'd like to ride this sucker sometime, it feels strong!
Thanks in advance for your knowledge.

[harisuluv]

The spec for your carbs uses a #40 idle jet if it is indeed a 76'.  That kit is probably a keyster and they put in #38 which is ok for F models.  Now, I had a similar problem and I rectified it almost immediately with the air screw. 

What is your air setup, airbox or pods, and what is your exhaust?  No one will be able to give you any advice without knowing these two factors to start from.

Now I would't have jumped to adjusting the needles right off the bat, and I am only assuming that cause that is how you stated what you did chronologically.  So, if you are running a stock setup, I would set the needles back and then fix it with adjusting the air screws. 

[Dinola]

Good points, and thanks.. The bike is a stock K, so 4 into 4 exhaust and the stock air box and air filter canister.  I have tried the air screw adjustment at every needle setting, grove 4, then 2 then 1.  The air screw definitely affects idle performance but has no influence over the continued running at high RPM.  I get this bike idling fine, rev it hard, release the throttle, see the linkage snap shut and then I can sit there and watch it just run like crazy at 5000 rpm until i intervene.
If i'm not mistaken, the 38 jet is slightly leaner than 40 but just slightly yet the bike runs like someone is pouring fuel in it.  I'll return the needle clip to a lower setting, for sure, but its keep revving almost independently of the clip setting.  I'm baffled.

[tlbranth]

If it's a stock bike, use stock jets and needles and set needles to the middle groove. Check your float levels.

[TwoTired]

#38 slow jet is standard for the 76 Cb550K.

Have you vacuum synced the carbs?
Have you check the mechanical timing advance for being "sticky"?

Have you checked for inlet runner leaks at the head and the carb couplers?

Is the air filter new?  (A must for carb tuning operations)

Have the mains emulsion tubes been cleaned"  (The main jet rests upon them.)

[Dimitri13]

Hanging idle suggests a lean pilot circuit. You must check for air leaks at all the couples, like TT suggested. You should replace the o-rings between the intake manifolds and head just for good measure. Uses the same o-rings as the tappet covers.

With your setup, you should be able to run perfectly at the stock settings (including the needle clip). Do your pipes have baffles in them or any rust holes?

[Dinola]

Yeah, Two, 38 was what was in the bike originally, so i thought it was right.  I've done all the other work you suggested except.. now you and Dimitri have me thinking about looking more carefully at the air intake path and the exhaust.  I do think i have the carbs right (after i reset the clips back to the middle groove) so its something else.  Timing advance seems to work right under a light.
Time to take a breath and re-examine the basics.
Thanks guys.