Topic: 1977 CB550 - Hanging Idle

[Keenankoso]

Hey all,

I know this has been pretty extensively covered on the forums in the past (I've read nearly every thread I can on this topic), but I was hoping to maybe get some others thoughts on this issue in case I've overlooked something.

I'm helping a friend restore his 1977 CB550 with PD carbs, and it's 99% complete aside from the fact that the idle constantly hangs. The bike fires up and runs beautifully on all 4 cylinders. It'll idle perfectly fine until any throttle is applied. If I've got the bike idling at 1400rpm as an example, but rev it to 3k, it'll slowly climb down and stick around 2k and not return to 1400. This happens no matter what the idle is set to.

Bikes specs and what we've done below:

- Carbs have been cleaned and inspected multiple times. Everything is to spec and nothing is blocked/plugged.
- Stock airbox
- 4 into 1 exhaust
- Brand new spark plugs
- We've tried both the stock points and a PAMCO electronic ignition. Issue persists.
- Battery is new and charged
- Checked intake and head for leaks multiple times to no avail
- Throttle is not sticking. Have done tests with the cable disconnected and operating by hand.
- Slides all move freely, no warping or debris inside carb
- Carbs have been synced
- New intake boots
- Tried idle mixture screw at .5 turns all the way to 5 turns out. Issue persists.

In my mind, it points to an air leak, but we've checked diligently multiple times and have yet to locate any. I had also thought that maybe one of the slow jets was plugged, but we pulled those last night to inspect and clean and found nothing. I had also read in another post that it could be running lean on the idle circuit, but we tested with the IMS backed out 5 complete turns and it didn't fix it. Like I said earlier, the bike starts and idles beautifully....until you rev it. The idle also won't return to where it was set if you let it sit, the only way to bring it back down is by backing off the idle adjustment.

Aside from the obvious air leak/carb culprits, could this be caused by an issue internally (piston rings, cam wear, etc.)? We did a compression test and found one cylinder to be slightly lower than the rest and performed valve adjustments which got it closer in line, so I don't believe that could be the cause but am willing to be proven wrong.

I took a short video in case this helps spark some ideas.

Thanks in advance!

[TwoTired]

If you reverse throttle pressure (Decelerate), what is the effect?

Is the throttle cable loose as in play before slides actually move?

If you put some restriction in the exhaust, like a bit of hand pressure, does the idle come back down?

Why did you set idle to 1400?

Is the bike fully warmed up when it does this?

Is your choke fast idle cam interfering?

What air filter choice is in the box?

Are you sure the pilot screw tips aren't broken off and stuck in the carb body?

Have you run this with the stock exhaust?  If so, were the symptoms the same?

Is there play in the slide lifter linkage?

When it idles high, are all the head pipes the same temp?

What method was used for carb balance?  Was the idle set that high for the balance?

[Keenankoso]

If you reverse throttle pressure (Decelerate), what is the effect?

Is the throttle cable loose as in play before slides actually move?
Throttle cable is properly adjusted and freely moving. Have tested without having it connected at all and manually raising the slides and the same issue occurs.

If you put some restriction in the exhaust, like a bit of hand pressure, does the idle come back down?
Haven't tested additional exhaust restriction, however, its a Delkevich 4-into-1 system with muffler and db killer, so there's plenty of back pressure as is.

Why did you set idle to 1400?
No reason. This was after adjusting the idle screw multiple times and was sitting at roughly 1400 when I took the video, so I used that as an example. Typically have it around 1k and problem persists.

Is the bike fully warmed up when it does this?
Issue occurs both when bike is cold and when fully warmed up. It was fully warmed up in the video

Is your choke fast idle cam interfering?
Haven't inspected this in depth, but this is plausible. Going to check this ASAP.

What air filter choice is in the box?
Unsure exactly what was put in there, but I'll check.

Are you sure the pilot screw tips aren't broken off and stuck in the carb body?
Positive. Had them all out last night to inspect. No damage.

Have you run this with the stock exhaust?  If so, were the symptoms the same?
No, stock exhaust was rusted away when the bike was purchased.

Is there play in the slide lifter linkage?
Nope. Functions well and slides move freely and easily.

When it idles high, are all the head pipes the same temp?
Yes

What method was used for carb balance?  Was the idle set that high for the balance?
Carbs were synced with a vacuum gauge tool. Idle was set at 1k for sync.

Thanks for the suggestions, TwoTired. I've got my responses in the quote. Going to check out the fast idle cam and make sure nothings sticking, but any other suggestions are greatly appreciated!

[david 750f]

In your video the choke is pulled out. Does it have a hanging idle with the choke off?

I had a similar problem and found it was the push cable (the one that actuates when you roll off the throttle). Disconnect the push cable and see if the problem persists.

Check that the advancer/springs are working.

[newday777]

Having been on multiple forums for the past 12 + years,  every time someone say they 'cleaned the carbs' or 'carbs are clean' my mind instantly says "How were t u e carbs cleaned" as in the method you used.....

What 'Sync gauge tool' did you use?
Have you verified each gauge is reading the same reading? As in have you tested them individually to one carb to verify that they read the same level? The cheap kits are notorious for being not set equal to each other and thus give poor carb sync results.