Topic: Help! '75 CB550 idles fast and inconsistent

[mireparo]

The bike: 75 CB550 stock except for pod filters. I cleaned the carbs (have cleaned many cb carbs before with great success). Rejetted for pod filters and synced them. This bike ran beautiful all last year and the beginning of this year.

The problem: One day after warm up (no change made prior to problem) the bike would begin to idle at 3000rpm. If I backed out the idle screw it would drop abruptly and die if not revved. I took it back to the garage and fussed with the idle screw a bit and had it idling at 1100rpm again so I went for another ride. The first light I stopped at it began to idle at 3000rpm. Again I backed out the idle screw slightly and it died!
I figured it must be a vaccum leak or the carbs needed to be synced again. Bought new carb insulators and o-rings for the manifold. Installed and re-synced; No adjustment needed. In the garage it seemed to idle fine, but again, on the first ride it began to idle at 3000rpm!!
I went over the throttle linkage and choke to ensure something wasn't sticking. Everything appears to be in perfect working order.
I have no idea where to go from here! As far as I know I've checked everything and don't understand where that extra air and gas is coming from all of a sudden!
Thanks for all your assistance!

[hymodyne]

bench sync (which you can do with the carbs on the bike, I think...

hym

[bwaller]

Does sound like carburation, maybe check the mechanical ignition advance to see if it needs to be lubed?

[mireparo]

bench sync (which you can do with the carbs on the bike, I think...

hym

Hym,
I'm not sure I understand. I believe bench syncing is done to get you close and then you still need to sync using a sync tool while the bike is running. The carbs are synced perfect at idle.

[mireparo]

Does sound like carburation, maybe check the mechanical ignition advance to see if it needs to be lubed?

Checked the ignition advance; No problems there. 

[bender01]

Could a float sticking cause this? Small piece of crap in the spring torpedo jet? (what is that thing called?)

[OldSchool_IsCool]

Leaky carb boots?

[mireparo]

Thank you all for the suggestions! Please keep them coming, there can't be much more to check. 

Leaky carb boots?

Just but new boots on it; Didn't make a difference

Could a float sticking cause this? Small piece of crap in the spring torpedo jet? (what is that thing called?)

I pretty sure the floats are working fine. If one was stuck open it would be dumping fuel out the overflow. If one was stuck closed that cylinder wouldn't get any fuel; All four cylinders are firing.
I'm not sure what your referring to as the "spring torpedo jet."

[Johnny5]

If the throttle cable isnt binding, I would guess the slow jets are the culprit.

[TwoTired]

The bike: 75 CB550 stock except for pod filters. I cleaned the carbs (have cleaned many cb carbs before with great success). Rejetted for pod filters and synced them. This bike ran beautiful all last year and the beginning of this year.

The problem: One day after warm up (no change made prior to problem) the bike would begin to idle at 3000rpm. If I backed out the idle screw it would drop abruptly and die if not revved. I took it back to the garage and fussed with the idle screw a bit and had it idling at 1100rpm again so I went for another ride. The first light I stopped at it began to idle at 3000rpm. Again I backed out the idle screw slightly and it died!
I figured it must be a vaccum leak or the carbs needed to be synced again. Bought new carb insulators and o-rings for the manifold. Installed and re-synced; No adjustment needed. In the garage it seemed to idle fine, but again, on the first ride it began to idle at 3000rpm!!
I went over the throttle linkage and choke to ensure something wasn't sticking. Everything appears to be in perfect working order.
I have no idea where to go from here! As far as I know I've checked everything and don't understand where that extra air and gas is coming from all of a sudden!
Thanks for all your assistance!

If it simply changed behavior after sitting with gas in the carbs, then it is most likely an idle jet issue.  Drop the carb bowls, remove the idle jets, clean them, and reassemble.

Another clue would be uneven head pipe temps.

[mireparo]

If it simply changed behavior after sitting with gas in the carbs, then it is most likely an idle jet issue.  Drop the carb bowls, remove the idle jets, clean them, and reassemble.

Another clue would be uneven head pipe temps.

It was running fine the beginning of this year. Just be sure I did drop the carb bowls and made sure all jets and passages were clear. Everything looked good. Head pipe temps all hot to the touch.
I even hooked up my sync tool again to see what it read while the engine was symptomatic. Still even.
Very frustrated!! 

[TwoTired]

There is no fast idle cam for warm up.  And a cold engine must idle at a higher slide setting than a warm/hot engine.

Are you adjusting the idle knob when hot or cold?  Or, both?

When throttle cables go bad, they can fray internally and hang up on the return or stick in various positions.
Do you have both the pull and "push" cables?  Have you removed the cables and checked them for free movement while flexed?

Could the slides be sticking in the carb bores?
Is your throttle return spring stock or a weaker replacement?

You sure you put the slide internal linkage back together correctly when you rebuilt the carbs?  The actuator arms tight on the torque rod?

Cheers,

[mireparo]

There is no fast idle cam for warm up.  And a cold engine must idle at a higher slide setting than a warm/hot engine.

Are you adjusting the idle knob when hot or cold?  Or, both?

When throttle cables go bad, they can fray internally and hang up on the return or stick in various positions.
Do you have both the pull and "push" cables?  Have you removed the cables and checked them for free movement while flexed?

Could the slides be sticking in the carb bores?
Is your throttle return spring stock or a weaker replacement?

You sure you put the slide internal linkage back together correctly when you rebuilt the carbs?  The actuator arms tight on the torque rod?

Cheers,

Cables are free and clear. I just double checked them and the carbs snap back closed quick on release. I can tell the carbs are hitting the stop even while it's idling high. As far as I know the throttle return spring is stock and plenty strong. I've had had experience with sticky and frayed throttle cables. Usually you can pull the throttle closed. Pulling the throttle closed makes no difference with this bike (and I'm 99.9% sure the slides are hitting the same idle position every time the throttle is released unless idle screw adjusted) Linkage is back correctly. I haven't taken it apart since the bike was running fine.

I know all four slides are always working in unison. (I hooked up the sync tool while bike was symptomatic and it showed them synced under all conditions)

I can start the bike, take it out warm it up and set the idle to 1100. Take it around the block and it'll idle at 3000 or more. Reset the idle to 1100, take it around the block and it'll idle at 3000 again...and again until the idle screw isn't touching! Then let it idle for 30 seconds and it will die and not start without idle turned up (understandably).

I'm so confused and frustrated!!
Thanks for advice and suggestions and please keep um comming!

Micah

[Ron_In_Conn]

I'm having a very similar problem with '81 CB750Custom.  Bike starts easily on choke, runs well, lots of power, etc., but when it's fully warmed up and I pull in the clutch, it'll rev out to 3000-4000.  I can sometimes hit the throttle hard, and it'll drop back to normal idle, around 1200.  I've greased up mechanical linkages, and can hear a solid 'clunk' when I release the throttle, so I'm assuming it's returning OK.  I haven't checked to see if head pipe temps are equal, but the bike is running strong, so I'm assuming that carbs are generally OK.

Possibly related information:  I re-sealed the gas tank, and dropped in an inline fuel filter, in case I screwed up.  Filter looks fine, and bike DOES run great at high throttle, so assuming not an issue.

Any thoughts from the learned assemblage? 

[RKilla]

Anyone have any luck with the above? I came across the same exact problem on my '78 550k and am scratching my head...

[Elkie]

I have as well. Not sure where to look next. 76 550k. Idle's good until a sudden rise to 3k rpm w/o touching throttle at all. Idle screw down to stop it then it bogs.

[MotoMartin]

I have the same problem it seems to be one or a combination of the following:
Worn or maladjusted throttle cable(s)
Sticking slides
Too weak, missing or incorrect throttle return spring
Air leak at carb boots
Incorrect float level(s) - look for leaking carb overflow
When (if) I fix it I'll post the reason.....

[the technological J]

i had this exact problem earlier this summer it was leaky carb boots (actually it was the intake manifold) and my timing was off... my vote is the timing very painstakingly do the timing with a timing light if u can( thier only like 15 dlrs) but check the carb boots by spraying some starting fluid in front of the carbs while running any change in  the idle  theres ur problem if it winds up being a leak.... i had to take the carbs off then the intake manifold then i got some copper based high temp gasket sealer put it on the rubber gaskets lightly snugged them up and tightened them down after letting it dry..... then my timing was off come to think of it i did it by ear... it settled on the higher idle then i just tuned it down with the idle screw..... good luck

[drivingslow]

I am still looking for a solution to the fast idle problem. 

All of the issues mentioned above, have been checked repeatedly, and are indeed correct on another 1975 CB 550. 

I was hoping someone else could address this issue...  Thank you.

[the technological J]

I am still looking for a solution to the fast idle problem. 

All of the issues mentioned above, have been checked repeatedly, and are indeed correct on another 1975 CB 550. 

I was hoping someone else could address this issue...  Thank you.

did u spray the boots and carb area with some starting fluid?... use sparingly.... if nothing then while the bike is running and warmed up... rotate the timing plate until rpms  are at thier highest (mine would go between 1100 and 4000.. when i did this it settled at 4000) tighten up the plate and turn down the idle... if this doesnt help then wait for two tired to chime in and maybe add a little more details so we know whats up

[edbikerii]

How about the o-rings between the intake manifolds and the head?  Those failed on my bike when she was only about 15 years old.

Again, the test is to spray the areas with WD-40 or starting fluid and watch for different combustion (faster or slower idle).

[Joshevelle]

Hello all,

I have the exact same problem, bike used to ride flawlessly, it is my daily ride and I have been dealing with this problem for three weeks now!

Setup: pods
Jets: 110 & 40
Spark plugs D7EA
Exhaust: 4into2
Stock engine

I already deep cleaned the carbs, new oil amd filter, new fuel filters (in line), new spark plugs, new air filters, but the problem persists.

I was suspecting of the right ignition coil because spark plugs #1 and #4 are getting carbonized, although I measured the same fuel level in all the carbs (#2 and #3 spark plugs are good). So I bought new points and condensers, swapped right coil with the left one, installed new spark plugs cables and boots. AND!!! The problem fu*%#ing persists!!! Spark plugs #1 and 4 are still getting carbonized in a couple of rides (see attached image).

Bike idles perfectly at my garage, but after a small three blocks ride, I struggle to keep it running and loses power.

I'm very frustrated right now, so any help would be highly appreciated.

Thanks a bunch!!
Josh

[BomberMann650]

Ditch the fuel filter and do the clear tube test!

Your jetting should be good for that setup.  What is your needle clip position and a/f mix screw set to?

[Joshevelle]

Hello bombermann,

I already did the clear tube test all bowls have the same level. Sorry but I didn't understand the reasoning behind ditching the in line filters, could you elaborate a bit more? bike  used to ride amazing and I've always used in line filters (paper, not stone).

[BomberMann650]

Sometimes the inline filters make for inconsistent fuel delivery. 

If the clear tube was good then move onto what the fellas have recommended.  Definitely inspect/renew/replace the carb rubbers.