Topic: Blarg. Still hanging idle... 1976 CB550K

[OldGreyBeast]

Despite my best efforts, hanging idle persists.  After putting the stock airbox back in (and spending about 3hrs fiddling with vacuum syncing) the bike it does run significantly better, cranks up immediately, usually only needs choke for a few seconds.  However it still has a tendency for the idle to hang.  Riding it around at very low RPM (1-3k) it does fine though obviously there's not much power down there.  Once I get the bike over 4k, it doesn't want to go under 4k.  It makes it very hard to slow down as I'm fighting the engine.  The only way to fight it down is to either choke the engine off for a few seconds or to just ride the crap out of the brakes and force it down to 2k or so.

Once it's idling at 4k, messing with the idle screw seems to do nothing.  It's like it just doesn't care.  I choke it for a couple seconds and it'll then idle along at 1200ish like it should.  It'll do fine again until I get it over 4k.

The bike is now all stock other than some cosmetic mods and a Delkevic 4-into-1 exhaust.

Help?

[fizzlebottom]

I haven't caught up on anything else you've done with this bike, and I'm more well-versed in the specifics of my CB650. Here's what I'd look at:

• How new and pliable are the carb insulators? Vacuum leaks will cause hanging idle.
• What jetting?
• Are they Keihin jets or aftermarket?
• Did you do the other adjustments like ignition, cam chain, and valves?
• Are your cables lubed and moving freely?
• Is your ignition advance assembly lubed and moving freely?

I know that might look like a lot of seemingly unrelated stuff, but let's just get it all out of the way before proceeding.

[OldGreyBeast]

Carb insulators were replaced late last year.  Airbox insulators are brand new.

Jetting is 110 (Keihin jets from jetsrus).  Idle jets are 40 because I just realized I didn't replace those when I had the carbs off this last time [rages internally].  I'm not sure if they're aftermarket or OEM.

It does have electronic ignition, timing has been set to the stock values from the manual.  Cam chain and valves have also been adjusted per the manual.

Cables are lubed and work freely.

Ignition advance assembly hasn't been lubricated but it does move freely.  I can oil it if need be, I wasn't sure if that was best left dry or oiled.

[Don R]

 One sticky slide can do that on my bikes but I have 750 K0's. Probably different due to the linkage.

[scottly]

Check the advance with a timing light (strobe). It should be at the "F" mark at idle, and only get to the full advance marks by 2400 RPM or so, and then return to the F mark at idle. If the springs are stretched or weak, it won't return to The F mark. If this is the case, the common fix is to cut one coil off each spring.

[OldGreyBeast]

I don't have a timing light, can you recommend one?

[Scott S]

 Why such big main jets with the stock air box?

[OldGreyBeast]

It has that 4 into 1 exhaust on it and ran lean with the pods and 110s, I figured the stock airbox would be enough to calm it down and maybe leave it a bit on the rich side.  At idle anyway the plugs look perfect.  I haven't done a chop at higher revs to see if it's rich when ripping or not.

[Scott S]

 The Delkevic really shouldn't need bigger mains.

[OldGreyBeast]

Would that prevent it from returning to idle?

[Scott S]

http://forums.sohc4.net/index.php/topic,184299.msg2133716.html#msg2133716

[OldGreyBeast]

I've read that thread before (that thread is part of why I pulled the carbs off and switched to the factory airbox), set the 49mm deal, etc.  My mixture screws are at 1.5 turns, I'll try setting those down to 3/4 like you did and see what happens.  I kept reading hanging idle was a lean condition but honestly I thought putting 110 mains in there would fix that.

Hopefully the mixture screws will do it... I'll try that after work today (if its not raining) and see if it makes a difference.  Mine is behaving exactly like yours did in the videos you'd posted in that thread.  It's not near as bad as it was with the pods but still just makes me angry and I look like a friggin fool having to futz with the choke and all that crap at every stop sign/red light.

[fizzlebottom]

Changing your main jets will not impact the idle circuit at all. Richen the idle mixture a tad and see if it improves. I'm not familiar with those carbs; do the idle mix screws control air or fuel?

[OldGreyBeast]

The idle mix screws on these control air.  I've closed them fully off and it makes no difference.  Totally taking them out makes the revs pick up a bit.  I have no idea what is going on with this thing.

The video below was taken with the idle screws turned all the way in.  I dunno why my phone filmed this sideways.

[TwoTired]

Do you know if the slides are able to reach it's travel floor in the carb body?  The individual adjusters can be skewed so that the idle knob not longer has proper effect.  Normally you asign A carb to be master and with the big idle knob backked out, adjust that carb so that it reaches all the way to the travel floor of carb float.  Lock that down and never adjust that one again, ever, adjust all the others to match, both mechanical sync and during vacuum sync.  After the master is locked down, then you set idle slide position completely with that idle knob.

There are stainless keepers on the adjusters 1-2 and 3-4.  These need to be flat and not bent so they introduce binding during throttle twist.

[OldGreyBeast]

I set the big idle screw to the 49mm deal that Scott's thread mentioned and used a 1/8" drill bit to do the bench sync. I'm pretty sure that the big idle screw should allow at least one of the slides to reach the bottom, but am not sure how I'd verify that with the factory airbox on there.

I did replace the keepers between the carbs with new, thicker ones from Classic Octane.

[OldGreyBeast]

I don't understand what would make the mixture screw not do anything other than some huge vacuum leak.  The intake o rings have been replaced, the boots are all new, like, where the heck would air get in from?  It didn't hang this bad with the pods, it's like the factory airbox made that part worse actually.

[Scott S]

 It's hard to tell in the video because you're moving the camera too fast, but what position is the choke lever in?
 It almost looks like your trying to run it with the choke ON. The lever should be pointing down for choke OFF.

[OldGreyBeast]

Some jackwagon cut the choke lever off this bike and welded a different one on it, so it looks weird.  For mine pointing parallel to the ground is OFF and pointing up in the air is ON.  I have to turn the choke on to get the idle to settle down.  Even with the choke on it takes a while for it to calm down.

[Scott S]

 I think you need to follow TwoTired's suggestion and baseline the slides.
 If a "jack wagon " has been in the carbs before, don't trust his work.

 Forgive me if this has been covered, but do you know the needle clip position? Are the needles Keihin or aftermarket?
 If you have the stock air box back on, you more than likely don't need a 110 main.

 But....before all that....have you done the 3k mile tune up? Valve and cam chain adjustment, ignition tune up and timing, etc. Carbs come last.

[OldGreyBeast]

No worries about asking questions, I'm glad to answer them.  Anything that helps.  I'm about ready to throw this thing on Craigslist.

I just rebuilt the carbs for the 3rd time a few weeks ago and used all Keihin brass.  Needles are in the middle position and have been replaced with genuine Keihin.

I fiddled with it some more after work and if I close turn the idle screw sufficiently enough the engine will cut off, so I'm pretty sure the slides can reach the bottom.

I did do the valve and cam chain adjustment about 1k miles ago.  Timing was set at this time, however I don't have a timing light to verify it while running.

[Scott S]

 Does the advancer move freely? Snap back into position? Have you tried trimming one coil off a spring(s) on the advance mechanism?

[beerme351]

I had a similar problem on my recently rebuilt k7.  Following all the steps in the shop manual cured the issue. 

If you've done these, I apologize, just getting caught up

Compression check(cheap tool you should own)
Verify spark plugs are good and set
Check timing (also cheap tool and you need to own)
Verify battery
Adjust valve clearance (every Honda owner needs to do this)
Adjust cam chain
Service air cleaner (you've done)
Clean and adjust carburetor

Mine was the carb and timing, thought it was prestine clean, but I had some rolled tiny o rings on the idle mix screws.  They hung up on my rebuild.  Agree with other members, no need to re-jet these carbs with stock airbox...I'd go back to stock.  Start from the top of the list, verify no vacuum leaks.  My bet is combo of jets, timing, and carb synch issue

[OldGreyBeast]

I had checked the advancer when I was messing with the engine last year but I'll check it again and make sure.  I haven't trimmed a coil off.  It has electronic ignition and was put on in '20 by the previous owner so I don't think the advancer will be worn out or stuck.

Can one of you guys recommend a timing light? They seem to sell for between $20 and $200.  I'm not against spending money on a good tool if it makes a difference, but don't want to waste money for no reason.

[beerme351]

I had checked the advancer when I was messing with the engine last year but I'll check it again and make sure.  I haven't trimmed a coil off.  It has electronic ignition and was put on in '20 by the previous owner so I don't think the advancer will be worn out or stuck.

Can one of you guys recommend a timing light? They seem to sell for between $20 and $200.  I'm not against spending money on a good tool if it makes a difference, but don't want to waste money for no reason.

I'm using the harbor freight and honestly have had good luck and no issues...https://www.harborfreight.com/timing-light-with-advance-40963.html