Topic: CB550 idle issue

[2tall]

Hey all,

After a week of spending nights cleaning on my carbs, I finally got the bike all back together (76 CB550k) and everything is running well. Except the idle. It just goes up and down as it wills between below 1000 and to about 3000 rpms. If I don't blip the throttle when stopped, it wants to die. Ideas?

Also, sometimes it is reluctant to idle down, but mostly in the first 5 min of riding. After that, it gets better, though not great.

I bench synced my carbs, and set the floats to spec. I turned the air screws out 1 1/2. What else am I missing?

Thanks in advance.

Paul

[TwoTired]

Vacuum sync the carbs.

[bistromath]

+1 what TT said, plus check for vacuum leaks on the carb boots.

[OldSchool_IsCool]

To check for vacuum leaks, start the engine, set the idle screw to a (relatively) stable RPM.  Spray carb cleaner on each boot that couples the carb to the engine.  If the engine picks up RPM when cleaner is sprayed, you have a leak.

[TwoTired]

If your bike has stock components, turn them to 1 1/8 or 1 1/2 turns out from lightly seated.

If your bike isn't stock anymore, the proper setting is what works well with the modifications made to induction and exhaust.

[d_soles]

Now, I'm no master at these carbs (cleaned a few in my day), but, boy oh boy, have I worked on Honda carbs.  Mostly scooters, but, actually, those old Sprees had very similar internals to these ones.  Did you make absolutely, positively, sure that you could see a pinprick of light through your idle jets?  Because, if they're clogged, that might be throwing things off a bit.  People just seem to clean them, and put them back in, never really checking how clean they are...also, I'm going to assume your emulsion tube was cleaned nicely (the brass tube with little holes, above your main jet)?

I know that it might be different on these bikes, but on the scooters I work on, many problems can be traced back to a carb that the owner said was cleaned, but wasn't as clean as he thought it was....

Doug

[flybox1]

i did the bench sync for my 350F, and had it spot on.
hooked up the morgan carbtune, and 2 and 4 we all over the place.
Vacuum was WAAAAAY OFF!!
got it sync'd and now idles, smothly, @ 1000 when warm. 


sync them already! 
fb1

[OldSchool_IsCool]

I have bench synced with both the 1/8 inch drill method and with TT's flashlight method.  I much prefer the flashlight.

Definitly make sure you can see light through all jets and tube orphices.  Also make sure you know how many hols there are supposed to be!  when they corrode over, they can be hard to locate.

[TwoTired]

Gee, I really hate to cut short your enjoyment of endless fiddling with bits of bike other than that which is causing your idle issues.  But, just in case you might want to experience some seat time focused on riding instead of tweaking....

Your carbs have slide adjustments on each carb.  It is possible to skew these adjustments so that the slides will never completely close (this is close to where idle occurs).  Here is another damnation of the "Drill Bit Method" where you can give yourself the false sense of well being that the slides are all matched physically, yet unable to close completely with the idle knob backed out.  It may also be a reason why later carbs have no adjustment on the #2 position.
FYI, adjusting them wide open ALSO does not ensure that all the adjustments prevent full CLOSURE of the carb slides.

When the individual adjustments are all skewed too far, the IDLE KNOB BECOMES INEFFECTIVE AT CLOSING THE SLIDES AND REDUCING IDLE RPM.  Oops, bumped th caps lock.
Back out the idle knob and align the bottom of the slides with them closed at the bottom of the carb throat. (Not wide open, as that is not the operation mode you are complaining about.)  This will get you a mechanical sync of the carbs, but not compensate for individual differences in the cylinder breathing efficiency (which is effected by valve lash clearances on the tune up schedule).

To get the carbs to make all the cylinders pull evenly at idle settings, a vacuum sync is specified by the Honda Shop Manual.
But, who knows, perhaps in the future, 2tall's book on "Adjustment procedure for balancing CB550 carbs" will make the best seller list?  After all, better wheels HAVE been invented over time.   



Cheers,