Topic: 76 cb550 with issues

[cargod78]

ok, i think i have some carb tuning issues and i am looking for verification and a place to start.  here is the story, i got a 1976 cb550 this winter, seems to run really well but lately it has started sputtering/popping in the exhaust only under part throttle it idles a little rough but accelarates beautiful with no hesitation or popping at WOT. it only seams to pop/sputter at 1/4-1/2 throttle feels almost like one cyclinder is cutting in and out.  it is stock except for the 4-2 open exhaust the previous owner installed.

the second issue is with idle speed at start up it doesnt want to idle, i have to turn the idle screw up, then once it warms up it idles too high so i have to turn it down, then sometimes it will idle low again when warm. basically i have to leave it slighltly high or sometimes it goes too low.  after a run on the highway 5minutes + at 70-80mph when i stop the idle goes very low and doesnt come back up till i have run around at lower RPM/speeds for a while.

[harisuluv]

Sounds like you might have multiple issues with your carbs.  Popping on deceleration if you experience it is usually a sign of running too lean.  An open exhaust system would support that.  What kind of air system do you have, stock or pods? 

It also sounds like you might have some sync issues too.  Do you know what your jet sizes are?

[cargod78]

stock intake.  the poping is on accelleration, when i am just getting into the throttle like the 1/4 - 1/2 range, above that it is beautiful feels almost like a cylinder is cutting in and out

[xsmooth69x]

sounds like it needs a full tune up

tappet gaps
points gaps
timing
clean carbs or rebuild it
sync carbs

[seanbarney41]

+1...don't touch those carbs until you have serviced your points/timing.  Factory service interval for this is every 1500 miles!

[Duanob]

3000 mile tune up is usually par for the course. Once all the timing items are set then go onto the carbs.

[iron_worker]

Agree with the others. Go through the 3000mile tune up then onto the carbs. Hard to diagnose problems when there are multiple problems going on.

Your description of hard start/won't idle without turning up the idle screw is a symptom of plugged idle jets. I know because I had the same problem ... Ran really rough when cold and needed lots of choke to get going. Wouldn't idle without playing with the throttle/choke/adjusting the idle screw up...a real interesting dance. As I drove it felt like I would go from 2 cylinders running to 3 and then suddenly 4 and it would run pretty well in the midrange and upper end.

I since did the full 3000 mile tune up (which helped) and then moved onto the carb rebuild. It was a bit scary at first but it you take your time and have nice neat work space it's not too bad. Clean all orificices thoroughly. Small particles of dirt/crud can easily cause problems in these carbs.

I found in my carbs I had 2 idle jets that were 100% blocked and a 3rd that was questionable.

I've recently got mine running again and sync'ed and it runs great. I'm very pleased.

IW

[cargod78]

Ok, thanks for the info. I guess I know what I'll be doing Saturday.


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[cargod78]

might just be coincidence but today i accidently left the fuel filler cap loose and the bike seemed to run a little better and idle seemed a little smoother..  any ideas?  plan on the timing/carb adjustments later today.

[cargod78]

so i went thru and gave it a little tune up, changed the plugs, oil and filter. set valve clearance, checked timing. i dropped the bowls and cleaned them out.  but it is still the same :  Idles fine, pulls good all the way thru if i stay on the throttle but it is when i am just easing into the throttle that i have issues.  it sounds like it is misfiring as i am easing on the throttle...  if i mash it, it is great but if i just gradually lean on it it kind of sputters till at least half throttle.  smae thing on cruising, if i am cruising at 50mph and want to just ease up to 60 it pops and sputters its way there, but if i go agressivly it is fine.  also surge on decel in gear.

[seanbarney41]

there is a vent in that cap...betcha it's plugged up

[DJ_AX]

there is a vent in that cap...betcha it's plugged up

+1

[cargod78]

there is a vent in that cap...betcha it's plugged up

unfortunately I tried it again today ( left the cap loose) and it ran exactly the same. Seems to run great all other times, just has part throttle issues


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[___aLovelyAntihero___]

My bike had similar issues... Rebuilt/synced the carbs, new clamps and intake gaskets (air leaks), new plugs, wires, coils, all made improvements, but what solved the problem was replacing and setting the points and timing. Some random electrical connections also had a nasty habit of coming loose. Just my experience.

[Dustinsandwich]

Hey, I am having the exact same problem. Funny thing is, I just upjetted for pod filters today, and I was having none of these issues before. I'm not super experienced, but it seems to me that would suggest some kind of mixture problem is the source? It's a 78 cb550 k3, by the way. Too tired to remember the jet size I've gone to. Any knowledge you guys could lay my way would help! Should I lean out with the needles or something? 

[XLerate]

Iron Worker got it right I think. To control air/fuel mixture & engine speed the individual carbs have 3 different/separate circuits: Idle, High Speed and WOT. Vacuum signals cause the carbs to transition between circuits, making the steps up or down to supply exact air/fuel quantity and mix as needed.

Clogged idle jets or/and air bleeds mess up both the idle running and transition to next High Speed circuit. There's an Emulsifion Tube on top of each Idle Jet that helps atomize and mix the air/fuel. The only right way to fix the problems for long term is to thoroughly clean carb circuits with carb cleaner and small wires.

If you can remove bowls without removing carb rack the brass emulsion tubes are visible, hanging down into bowl area. These have crosswise holes in them plus a hole in the domed tip that need to be cleaned with wires & carb spray cleaner, best after soaking them. May be able to remove each one without pulling carbs, not sure on that model. Also need to clean air bleeds.

I wrote general instructions posted in this thread if you want to do a complete carb clean:

http://forums.sohc4.net/index.php?topic=110161.0

May get lucky and clever and be able to shortcut some of that, depends on several things. Carb cleaner & jet cleaning wires are the secret to happiness for your carbs one way or the other.

[seanbarney41]

Vacuum signals cause the carbs to transition between circuits, making the steps up or down to supply exact air/fuel quantity and mix as needed.

this is incorrect, only throttle position opens, closes, and transitions between circuits...these are not cv carbs   i.e. whether low, mid, or high speed circuits are in operation or not is completely dependant upon throttle position.

[XLerate]

Vacuum signals cause the carbs to transition between circuits, making the steps up or down to supply exact air/fuel quantity and mix as needed. Also the steps between circuits or transitions depend on the throttle butterfly or slide piston to effect the different circuits and their vacuum signals in order to maintain a stochiometric air/fuel mixture throughout operating range while varying available quantity of same according to engine demand.

this is incorrect, only throttle position opens, closes, and transitions between circuits...these are not cv carbs   i.e. whether low, mid, or high speed circuits are in operation or not is completely dependant upon throttle position.

I had no idea these were purely mechanical carbs, never heard of carbs like that before. I'll be darned. Also never heard of carbs where Idle or High Speed circuit opens and closes.

All the carbs including Honda multi's I've ever worked on have 'air bleeds' in the carb throat inlet, inside carb throat, at venturi etc. that give  low pressure 'vacuum signals' as needed to draw fuel and mix with air. Also the Idle circuits and High Speed circuits never close as such but only attenuate according to demand. The variance in high/low pressures cause the carburetor to maintain a constant stochiometric fuel/air mixture balance across the rpm band regardless of engine demand. Looks to me like these Honda carbs are like that too. Every other Honda carb I've seen was and so was every other manufacturer's. The CV carbs I've worked on use a vacuum diaphragm to adjust throttle position. Are you saying these operate by a high pressure ram effect instead?

[seanbarney41]

air bleeds are uncovered by opening throttle

[XLerate]

air bleeds are uncovered by opening throttle

Are you saying the air bleeds are in the slide piston chamber? Honestly, I think you're confused on carburetion theory. Here's some information that might help:

http://www.dansmc.com/carbs2.htm

[seanbarney41]

no, I guess I was speaking more figuratively, the slide raises the needle and simultaneously allows more air to draw fuel through.  We are argueing the same thing here, just with different words.   Let me re-state; vacuum does not cause the slide to raise, your hand twisting the grip does...cv carbs; vacuum DOES cause the slide to raise... OP needs correct info to solve his problems, not a couple of know-it-alls discussing symantics

[XLerate]

no, I guess I was speaking more figuratively, the slide raises the needle and simultaneously allows more air to draw fuel through.  We are argueing the same thing here, just with different words.   Let me re-state; vacuum does not cause the slide to raise, your hand twisting the grip does...cv carbs; vacuum DOES cause the slide to raise... OP needs correct info to solve his problems, not a couple of know-it-alls discussing symantics

I've been riding, tuning and rebuilding motorcycles for about 47 years so yes, I already figured out that the twist grip controls engine speed if the engine is running. I never said anything in what I wrote that vacuum controls throttle position or causes slide to rise, except where I referenced CV carbs in the Carb Cleaning article. Speak for yourself about know it alls discussing symantics! You gave misinformation that can have him all screwed up understanding basic carb theory. You're taking pot shots at what I wrote trying to show errors that aren't there because I didn't even address the area you reference! Instead I gave true, accurate and correct information.

The whole carburetor throat is a relative low pressure vacuum compared to atmospheric pressure and it is this vacuum which draws fuel through jets and emulsion tubes and mixes with air bleed pressures to mix and atomize air/ fuel to deliver to carburetor throat and into engine intake. If there's a clog or restriction in any of these parts or circuits the entire system will be imbalanced. The delicate transitions between circuits caused by differing vacuum signals and air bleed pressures are what controls the overall air/fuel mix delivered to engine. The throttle slide piston adjusts the amount of low pressure vacuum that flows through caburetor throat to enable functions above Idle speeds.

Here's another link to a more thorough explanation of all functions occurring in a carburetor:

http://roadstarclinic.com/content/view/56/96/

[seanbarney41]

read it again dude...I said a COUPLE of know it alls...no knowledge slight intended, let's just be friends.  Sounds like you have way more experience than me.  I'll let you solve the op's questions.

[XLerate]

read it again dude...I said a COUPLE of know it alls...no knowledge slight intended, let's just be friends.  Sounds like you have way more experience than me.  I'll let you solve the op's questions.

Sounds good, I'll go for that.

[Dustinsandwich]

So... what I'm thinking is when I upped the main jets I needed to up the pilots too, or at least enriched the idle screws. Does this seem correct to you guys? Forgive my inexperience.