Author Topic: Rejetting the carbs?  (Read 3534 times)

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Offline 78_SaltLick

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Rejetting the carbs?
« on: April 03, 2006, 08:53:58 PM »
Hello again.

So i got a 74 CB550, was having a terrible time spitting and sputtering, and blowing out black exaust.  i gave it a tuneup, replaced the spark plugs, and points. Ran a little better but still coughing. i then took off the air filter, and ran it with no air filter. it ran like a dream! This bike has a 4 into 1 straight pipe exaust, someone told me they thought what the deal is that who ever put the pipes on didnt rejet it for those pipes. I have no idea what that means. They said i need to rejet it so it will run right with a normal air filter in it. I figured since the air filter was really old, id replace it with a new Uni filter, that would make it run good, and while it did improve the performace alot, it still spits and sputters a bit, especially after it gets warmed up. All in all, the tuneup/new air filter sure did make a big difference in how much it spits and sputters, it was almost unrideable before that, but man if i take off the air filter away it goes! not one cough, not to mention the speed and power of the bike about doubles. ...So... question is how do i rejet the bike so it runs right with this filter on it? I have the online 550 owners manuel, is it in there? Any help would as always be appreciated.
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Offline 78_SaltLick

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Re: Rejetting the carbs?
« Reply #1 on: April 03, 2006, 08:59:02 PM »
Oh yeah, i forgot to mention this, dont know if it means anything. The new uni filter, the hole in the filter is a little larger than the stock filters hole, and i am missing the bracket that holds the filter in place, so the filter if you put it in there by itself left a little gap, so i crammed a piece of wood down in there to keep the filter right against the hole. Well, its crammed in there pretty tight, i mean the air filter is really pushed in there snug, so are these suppose to be a little loose? Possibly its choking the carbs with the way i have it smashed in there? lord i dont know.....
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Offline TwoTired

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Re: Rejetting the carbs?
« Reply #2 on: April 03, 2006, 11:19:11 PM »
Given what you've told us so far,  I think there are two things left to do.

Even new spark plugs can become fouled and misfire. If you look at your new plug's center electrode insulator and it is covered in black soot, this is probably the cause of your spit/sputter.

If it got that way because of the old clogged up filter you ran with the new plugs.  You might be able to get away with cleaning or replacing them with new.  Then, if you haven't over oiled the new foam filter, your problem might be solved.

If, however, the cleaned or new plugs soot up again rapidly, I would then lower the slide needle in each carb (throttle valve) one notch to lean the midrange mixture ratio.

The needle adjustment is a task that requires some skill and dexterity.  And, it is mandatory that the carbs be synchronized after the proceedure.  It is possible the adjust needles on the 74 CB550 without carb removal, but not easy.  See page 58 of the honda shop manual for extracting the slides.

You might also be able to lean the idle mixture by turning the air bleed screws out on each carb 1/4 to 1/2 turn.  However, if your throttle response from low speed deteriorates, you'll have to turn them back in a little.

Good luck!
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

BruceA cb550

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Re: Rejetting the carbs?
« Reply #3 on: April 04, 2006, 08:31:21 PM »
would pod filters help if its running rich?

eldar

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Re: Rejetting the carbs?
« Reply #4 on: April 05, 2006, 07:34:22 AM »
Oh now that would be a whole new world of trouble, trying to run pods.

Offline crash

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Not trying to hijack the thread but.....
« Reply #5 on: April 05, 2006, 10:18:59 AM »

Question on pods and jetting.

Pods are on my 73 750 engine.  Carbs were supposedly cleaned and calibrated when I bought it.  It runs rough, won't idle below 1600, and when I goose the throttle to accelerate it just wants to die at 4000 rpm.  So two things:

1 - I have been told that the vacuum which exists with the airbox is necessary to make the carbs work properly.  Pods just can't achieve the vacuum.  Yet I constantly see guys running 4 into 1 exhaust, pods, etc. and they keep saying everything works fine.  Other guys seem to be readjusting their carbs constantly.  So, is it possible to say once and for all that the airbox should not be replaced?

2 - Currently have 110s on the carbs, was told to go to 120s.  But I just got thru a long discussion with a guy who does mostly old British bikes.  He went into a long diatribe about gas not being the same as it was 30 years, what with all the detergents, etc, etc.  He said he recently had a problem with an old bike where he ended up going down to 95s.  His rational was sound, but I am not 100% convinced.  Is there anyone out there running main jets less than 100s?
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Offline dusterdude

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Re: Rejetting the carbs?
« Reply #6 on: April 05, 2006, 11:27:57 AM »
no.1,your first paragraph is bs,i run pods and open 4 into 2`s,i have no issues other than i need to re-jet which i havent done yet.
mark
1972 k1 750
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Offline TwoTired

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Re: Not trying to hijack the thread but.....
« Reply #7 on: April 05, 2006, 11:46:09 AM »
Pods are on my 73 750 engine.  Carbs were supposedly cleaned and calibrated when I bought it.  It runs rough, won't idle below 1600, and when I goose the throttle to accelerate it just wants to die at 4000 rpm. 

First, tell us that you have done the entire tune-up checklist and the air filter is new or so clean that it behaves as new.  Then balance the carbs.  If troubles still persist, then address carb mixtures.

1 - I have been told that the vacuum which exists with the airbox is necessary to make the carbs work properly.  Pods just can't achieve the vacuum.  Yet I constantly see guys running 4 into 1 exhaust, pods, etc. and they keep saying everything works fine.  Other guys seem to be readjusting their carbs constantly.  So, is it possible to say once and for all that the airbox should not be replaced?

Of course not.  However, if you expect the factory settings for the carbs to work as delivered from Honda, then yes, the stock airbox, filter, and exhaust are the proper combination.
A filter that changes restrictive properties, effects the carb throat pressures relative to the surrounding barometric pressure.  The differential pressure is what pushes or draws the fuel from the metering devices in the carburettors.   More restriction draws more fuel through the jets for any given RPM or throttle setting.  Restriction varies with filter material, amount of foreign debris trapped by the filter, and the turbulence signature relative to the velocity of air moving through it.   The latter can be a non-linear relationship with velocity.

2 - Currently have 110s on the carbs, was told to go to 120s.  But I just got thru a long discussion with a guy who does mostly old British bikes.  He went into a long diatribe about gas not being the same as it was 30 years, what with all the detergents, etc, etc.  He said he recently had a problem with an old bike where he ended up going down to 95s.  His rational was sound, but I am not 100% convinced.  Is there anyone out there running main jets less than 100s?

Unless you are using fuel blended with alcohol, it has the same energy content it always did.  Detergents are added to keep the fuel injector nozzles clean, as well as the entire fuel system.  The octane rating only changes the flame front travel speed.

For the home mechanic/motorcycle modifier who does not have access to a Dynomometer and exhaust sniffing equipment, learning to read the spark plug deposits is essential toward rational and straightforward adjustments to the carburetor's multiple metering systems.  A track and a stopwatch are also helpful for peak tuning.  Some knowledge about how your carbs work and what metering system is dominant at a given throttle setting is also helpful in minimizing the carb adjustment flail after changing your bike's looks with intake and exhaust changes from stock.
I don't believe in all the "rules of thumb" for carb jetting.  Component characteristics vary among manufacturers.  The way to tell what your bike in its current configuration needs, is to read the spark plugs and note the conditions and throttle settings under which the plugs were operated.  Then address that aspect of the carb that governs those conditions and adjust accordingly.  

The old brit bikes don't acheive the high RPMs the Honda fours do.  The main jet is set for wide open throttle and lots of air velocity through the carbs.  Velocities the old brit bikes can acheive only very breifly before shedding important internal bits.   Your midrange issues will likely be addressed with throttle valve (Needle jet) changes, and your idle issues with either air bleed screw or slow jet changes (or both).

Learn to read plugs here:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Here is a chart as to what to adjust in the carb.  Good luck!




« Last Edit: April 05, 2006, 11:52:41 AM by TwoTired »
Lloyd... (SOHC4 #11 Original Mail List)
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Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline seaweb11

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Re: Rejetting the carbs?
« Reply #8 on: April 05, 2006, 11:47:21 AM »
Duster, put in 138s and ride with the wind.
I have the same set up as you, I may still go to 140s some day/

Offline petersan

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Re: Rejetting the carbs?
« Reply #9 on: April 05, 2006, 12:57:11 PM »
I understand the idea behind reading sparkplugs - but what is the minimum amount of time the bike needs to run at a particular throttle setting to get a good reading?

Say I want to know the reading at idle - do I install new plugs, start the bike, and let idle for 2 min or 10 min?

Next I want to check if main jet size is good - do I install another set of NEW plugs, start bike and run it open it up for a minute or more? 

I guess I'm asking - if it is rich at idle, but lean at WOT, will the dark deposits on the plugs (from idle) burn off when the throttle is fully open?

Offline Bodi

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Re: Rejetting the carbs?
« Reply #10 on: April 05, 2006, 01:36:48 PM »
The plugs - unless horribly carboned up or burnt (melted) will show true after about a minute or two. You don't need new plugs each chop.
Generally you should check WOT first, this can be a bit dodgy unless you have a long uphill stretch of highway available, you'll be above the speed limit I would hope. Go flat out as long as you dare, then switch the kill switch OFF and pull in the clutch, go down through the gears as you slow down but keep the clutch in. Then have a look at a plug or all of them.
You want to get the main jet right first, futzing with the needle is pointless if you aren't sure the main is right: the main will affect the mid/high throttle mixture and your needle setting will be wasted if you have to adjust the main later.
Then I would check the mixture at about 1/2 WOT - you should look at the slide position and mark your twistgrip position for an accurate setting while riding.
That should get your needle setting pretty close, last set the low speed screw for highest idle and see if the engine runs well. Further checking at 1/4  and 3/4 throttle might show you a need to readjust or even modify the needles.
Note that the needle meters fuel through the emulsion tube orifice, not the main jet. The orifice can wear after lots of miles or if the needle has been damaged and was rubbing in the orifice, that makes for big trouble trying to tune.

phylo101

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Re: Rejetting the carbs?
« Reply #11 on: April 05, 2006, 04:51:15 PM »
Hey, think about this next bit for a moment before you do anything else.......

Do your plug chop with your filter OFF......

What were the plugs like? She ran fine, are they fine?

The reason Im saying that is Ive found over the years that for SOME odd reason the jetting on 550s is strangely generous in leeway to either side of making any changes in the induction system before you ned to rejet. My old 550F2 ran open bellmouths, an open 4 into1 pipe...and ran PERFECTLY on stock jetting! Which I found VERY odddddd

My bud had an almost identically modded bike, except he ran proper KandN pod filters, properly oiled and all, and ran main jets a size larger.....or rather thats how the bike came to him......

Now - at one point he encountered a problem with a float hanging up, and as my bike was garaged for a while he borrowed my carbs.....and HIS bike ran FINE, exactly the same and as reliabley with MY carbs on it with their STANDARD jetting.....

I know thats totally against ALL the rules, but do your plug chop and prepared to be suprised at the results......

phylo

Offline 78_SaltLick

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Re: Rejetting the carbs?
« Reply #12 on: April 21, 2006, 11:55:37 PM »
Given what you've told us so far,  I think there are two things left to do.

Even new spark plugs can become fouled and misfire. If you look at your new plug's center electrode insulator and it is covered in black soot, this is probably the cause of your spit/sputter.

If it got that way because of the old clogged up filter you ran with the new plugs.  You might be able to get away with cleaning or replacing them with new.  Then, if you haven't over oiled the new foam filter, your problem might be solved.

If, however, the cleaned or new plugs soot up again rapidly, I would then lower the slide needle in each carb (throttle valve) one notch to lean the midrange mixture ratio.

The needle adjustment is a task that requires some skill and dexterity.  And, it is mandatory that the carbs be synchronized after the proceedure.  It is possible the adjust needles on the 74 CB550 without carb removal, but not easy.  See page 58 of the honda shop manual for extracting the slides.

You might also be able to lean the idle mixture by turning the air bleed screws out on each carb 1/4 to 1/2 turn.  However, if your throttle response from low speed deteriorates, you'll have to turn them back in a little.

Good luck!


i pulled my plugs tonight, so i took a look at the plug pics on the chart. plugs seem to be okay. on the end the little nub is black with soot, but the houseing around it? (white part) is a light chocolate brown like it suggests in the pics as good. This is about a month of riding after replacing the plugs. Is that little nub suppose to be white as well? I thought a plug covered in soot on the end meant you were fouling your plugs.
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Offline TwoTired

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Re: Rejetting the carbs?
« Reply #13 on: April 22, 2006, 01:08:08 AM »
i pulled my plugs tonight, so i took a look at the plug pics on the chart. plugs seem to be okay. on the end the little nub is black with soot, but the houseing around it? (white part) is a light chocolate brown like it suggests in the pics as good. This is about a month of riding after replacing the plugs. Is that little nub suppose to be white as well? I thought a plug covered in soot on the end meant you were fouling your plugs.

At the center of the plug is a metal electrode.  The white part is a ceramic electrical insulator.  When you read plugs, you are looking at the deposits.
Soot is a black powder that rubs off easily.  Simply darkened metal is not soot.  (Although, soot can become glazed)  The plug's electrodes should be the hottest part of the combustion chamber, as it is where the burn initiates.  It should be the last thing to have soot on it in a running engine.  It makes no sense to have soot at the electrode tip metal and not on the white porcelain insulator.

A fouled plug can have a bit of carbon lodged between the electrodes preventing a spark arc.  Or, it can be a center electrode insulator covered in carbon such, as soot, that bleeds off the spark voltage before it can jump across the electrode gap.  Both conditions effectively short out the plug and prevent it from igniting a fuel /air mixture.

Post a detailed, in focus picture of a characteristic spark plug from your engine.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline eurban

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Re: Rejetting the carbs?
« Reply #14 on: April 22, 2006, 06:48:13 AM »
I have read thru this thread but I am wondering why a mildly modified setup would run as poorly as you describe with the airbox on? Seems to me that before you spend your time with all the nuances of carburetor fine tuning you need to be pretty sure that all the bikes systems are working just right, and particularly that your carbs and intake setup are up to snuff.  It may be that the 550 is very sensetive to a swap in exhaust but if the carbs are really working correctly, set to stock specs and your replacement air flilter/wood assembly is not restricting airflow unecessarily then I would gamble to say that the bike should be doing better than you describe.  I wonder about . .  .a restrictive airfilter/block of wood? non stock main jetting, needle position, idle jets and mixture settings? choke operation? float level?, worn needles (main jet needles)/ needle jets?  and if the bike is otherwise in proper tune with a strong/timed ignition in particular?  I remember spending most of a summer a while back trying to fine tune my 750s carbs only to eventually figure out that my idle jets were pretty much clogged.  Only thing I gained  was a bit of experience, the tuning process had to start all over again.  Hope this helps
« Last Edit: April 22, 2006, 06:49:45 AM by eurban »