Topic: CB550 with a random high idle

[AaronJ]

So i just got my 1977 cb550k running good.  It took a good carb cleaning and some new plugs, and now it runs great.  When i first started it and let it run for a bit, i set the idle a little above 1k, after a bit of riding i will pull up to a light and the idle will be pretty high, at 2500 or so...but if i let the bike sit for 15-20 seconds the idle backs down again and is the same.

Things to note.  I don't have the valve cover breather routed back into the airbox anymore (i'm installing pods soon anyways) and i haven't done any work on the ignition system yet either.  Also, the bike doesn't have the stock pipes anmore, just drag pipes, but it pulls good and doesn't seem lean at all.

Could richening up my idle screws change this at all?

[754]

Generally only 3 things,

Check at your idlesrew ajustment, that it is returning everytime..it may be sluggish and not always closing the last tich.

If you eliminated that,

then it it probably an intake rubber air leak... or the advance sticking..

[TwoTired]

Have you vacuum synced the carbs?

What do your plug deposits look like?

What have you set your IDLE Mixture Screws at?

How is your throttle response from low RPM by opening the throttle under load?  Can you make a 1/2 throttle travel movement and get good pickup?

What air filter are using, associated with with your current performance issue?

Do you recall what pilot jet size you have?

What position is your throttle valve needle clip in?

Cheers,

Here's my guess, without the information from the questions (whose outcome can destroy accurate guessing):
You are running lean on the throttle valve needles.  Making an initial lean condition on return to idle.  The idle system in the carb gradually takes over and provides a proper mixture for idle.  When that eventually happens, the idle returns to normal.

[hapsh]

I had the same thing on mine a few months ago.  After doing a vacuum sync I found that two of the cylinders were way to high vacuum and the other two were two low. This made me adjust the idle stop screw high enough so that the low vacuum cylinders were keeping the engine idling but they were in the needle jet range of throttle while the other cylinders were almost not running.  Once I balanced the cylinders they all were running off the idle circuit and staying stable.  I would also check your timing advance as well.  These old bikes tend to get weak return springs on the advancer, when the bike warms up the springs don't quite bring the advance mechanism back to its resting position.  That will cause the idle to hang high.