Topic: In Carburetor Purgatory with my 1975 CB550. Penance needed please.

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Summary:

I've been to hell and back with the carbs on my 1975 CB550 and find at what can best be defined as purgatory (half way there) at this juncture as it relates to getting her synched at book value (1200) rpm and must have the choke slightly open for her to stay running. 

Background:

Bought the bike a couple weeks ago and have just finished rebuilding the carbs to include new carb kits throughout, soda blasted exterior "with orafices covered" and then blew out the interior orafices with a combination of  compressed air and carb cleaner and then confirmed clean passages individually throughout.  For the carb kit I am running  100 main jets, 38 slow jets, needle jet e-clip at second ring up from bottom on the 5 detent needle and fuel/air screws out at 1.5 turns and a float postion of 22mm.  Again, all book SOHC-Forum recommended values.  Air box is stock, clean and has no leakage as I again confirmed this via spraying starter fluid at connecting passageways to check for potential surge/drop while running.  The only slight modification to the bike comes in the form of a two-into-one Jardine exhaust that is in good condition including the baffles.

Prior to rebuilding the carbs (bike ran sorta ok before the rebuild of carbs), I adjusted the tappets (book values), sanity checked the points and timing, and ensured that the cam-chain tension was dialed in.  Using my newly purchased Harbor Freight compression tester, I report an average reading of 130psi across all cylinders with only a +/- one percent difference between them.  Good to go on the top end.  Replaced plugs with SOHC Forum recommended NGK plugs for this mostly stock setup but cant recall exact plug serial number (I'm at work at the time of this SOHC prayer session) but report that they were a nice golden color prior to the carb rebuild/synch and visually verified spark from each plug and associated wire prior to the re-synch fiasco I now find myself in.  Coils are original...its a sin for the price they want for these things.

Discussion:

I am able to get to equal values anywhere between 16 to 24 on my mercury based, 4 tube motion pro manometer, but have to have the choke slightly open at all times in order to keep her running.  After getting level readings across all four carbs on the manomenter (with choke slightly open to stay running), I then tried increasing the main idle screw while decreasing the choke by equal amounts  in an attempt to eliminate the need for the slight choke requirement she currently has, but report "no joy" at this time.  A superfan of the forum, I then went back and read nearly every single carb post in the FAQ and employed the 1/8th inch drillbit benchmark trick in the throats of each slide to establish a solid baseline prior to my third and final round of trying to eliminate the need for slight choke to keep her alive.  Needless to say, I got TKO'ed after three rounds and am on my knees at my seat wondering what I've done wrong as I typically like to find fault with myself before components, which by-the-way, keeps money in my wallet more times than not.   I have had the complete air box installed while conducting the synch.  Is this wrong of me?  Can I conduct a proper synch without the air box on?  I did not change jet sizes...should I have?  Could I have a weak spark across the spectrum perhaps?  Too lean...Too rich?  The list goes on and on.....

I appologize in advance for my diatribe but saying hail mary's and reflecting on the matter wont get her to stay running (albeit badly) on the first kick like she did prior to my shenanigans.  Please be nice, I work on the bike to calm PTSD vice trigger it.

[orange550]

I'm not nearly as experienced as many here. A few things you might check. (also, needing a bit of choke means maybe it's too lean, yes?)

- check the vent on the gas cap. make sure it allows air into the tank (mine was plugged and caused me issues)
- petcock screen plugged? my tank leeches a bit and I need to check my filter often.
- fuel line issue?
- did you pull out the emulsion tubes on the carbs? I rebuilt my carbs a couple of times before I figured this one out
- overflow tubes on float bowls plugged? I think they need to be clear to allow the bowls to fill up?

Just a few ideas.

[matt mattison]

-gas tank clean and rust free?
-check the intake o-rings for vacuum leaks?

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Thanks for the speed of need responses thus far.

Yes, a lil choke is, for the most part, indicative of a lean condition (too much air - not enough gas)
Yes, fuel is flowing freely from petcock.
Yes, tank is rust free.
Yes, overflow tubes on float bowls are clean.
No, I didnt check gas cap vent.  Great Idea.
No, I didnt check intake o-rings for vacuum leaks given that the issue started in such short duration.  Worth investigating though
No, I overlooked the emulsion tubes.  Further SOHC forum research indicates "Insane Reindeer" had similar issues that tied back to his emulsion tubes.  +1 orange550!  Rookie mistake on my part.

[lucky]

Here are the essential facts of this saga.

1975 CB550.
Main jets #100 mm.
idle jets #38 mm.
Slide needle clip position 4th from top.( Always say from the top)
Mixture screws 1.5 turns out.
Float height 22 mm.
Exhaust 4 into 2 Jardine with mufflers.
Intake stock air box.
 (You must have the air box in place to sync carbs because it effects the mixture.)

Learning is always a struggle but what you learn will be with you for life.

Basically this motorcycle has all stock parts except the Jardine exhausts.
I do not know if the float needle and seats were replaced, but if they are not NEW it could cause trouble from now on, and torture you more.

I do not see how this bike can run right.
There has to be some kind of change to make up for that Jardine exhaust.
A mixture adjustment is the place to start.
If you turn the mixture screws to make it richer and it still hesitates or has bad throttle response(warmed up) unless the choke is on, then I would go up a step on the main jets and I would go up on the idle jet one step too.
I would go to #105 mains and #40 idle jets mixture screws 3/4 turns out.

BTW when all of you get some results then never report back, you have just cheated all of the forum members out of any help- like you got.
ALso it makes it harder for me to help you.
I am keeping records of all the bikes and exhaust systems and jetting and the more records I get the faster I can help you and you go through less hell and expense.

This is probably the 4th one of these bikes this week and no one has reported back.

[dave500]

you had the carbs off and didnt pull and clean the emulsion tubes?do them again.
 

[mono]

sounds like you've been thorough, but what do you mean by "sanity test" on the points/timing?  just haven't heard that before.

assuming you're going with stock timing positions and the points are gapped properly, other suggestions would be to increase the jet sizes.  i have also gone through everything on my bike (same year 550) and have the same issue after installing a MAC 4-1 exhaust.  just ain't getting enough juice, and so i have to leave the choke open just a tad to keep her going smoothly.  everything i've found points to needing larger jets - as i expected i would - (40/110s are what i've decided to order, based on a number of suggestions by people with the same setup as me), but you may just get away with an upgrade to 40 and moving your needle position.

[lucky]

sounds like you've been thorough, but what do you mean by "sanity test" on the points/timing?  just haven't heard that before.

assuming you're going with stock timing positions and the points are gapped properly, other suggestions would be to increase the jet sizes.  i have also gone through everything on my bike (same year 550) and have the same issue after installing a MAC 4-1 exhaust.  just ain't getting enough juice, and so i have to leave the choke open just a tad to keep her going smoothly.  everything i've found points to needing larger jets - as i expected i would - (40/110s are what i've decided to order, based on a number of suggestions by people with the same setup as me), but you may just get away with an upgrade to 40 and moving your needle position.

I am glad you posted.
The avenue of raising the needle by lowering the clip to the fifth groove down from the top and increasing that idle jet could be an excellent way to go .

Please let us know if that got you good results. THANKS!!!

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Copy all....I'm going to go with lucky on this one and see if #105 mains and #40 idle jets will alleviate the issue.  I will most definitely provide feedback.  Collaboration is a two-way street and know full well that you dont have to have gray hair to be a wise man and am thankful that god gave me ears that are always open and a mouth that can close (aka listen more than you talk).  As a rookie to the CB I now know that the component terms "emulstion tube" and "needle jets" are one in the same.  So yes, these are new and clean.

Hats off to the collaborators above! 

Cut

[orange550]

....

BTW when all of you get some results then never report back, you have just cheated all of the forum members out of any help- like you got.
ALso it makes it harder for me to help you.
I am keeping records of all the bikes and exhaust systems and jetting and the more records I get the faster I can help you and you go through less hell and expense.

This is probably the 4th one of these bikes this week and no one has reported back.

Someone not reporting back doesn't mean they fixed the issue. I wouldn't be surprised if a lot of guys just give up or live with the problems.

[lucky]

....

BTW when all of you get some results then never report back, you have just cheated all of the forum members out of any help- like you got.
ALso it makes it harder for me to help you.
I am keeping records of all the bikes and exhaust systems and jetting and the more records I get the faster I can help you and you go through less hell and expense.

This is probably the 4th one of these bikes this week and no one has reported back.

Someone not reporting back doesn't mean they fixed the issue. I wouldn't be surprised if a lot of guys just give up or live with the problems.

That would be sad.

[TwoTired]

I don't believe there is any need for larger jets.  There is certainly not enough info so far to make such a determination.
And, the present configuration of the bike is such a minor deviation from stock that jetting changes are unwarranted, imo.

Certainly clean the emulsion tubes.

Choke terminology:
Choke butterflies closed, equals choke "ON".  For the 022A carbs, (which is what a 1975 Cb550K model would have had leaving the showroom) choke on is lever up position for this condition.

The choke increases throat pressure differential, which forces more fuel from all jets while blocking air intake.
After warm up, no choke should be need, and the lever should be fully down.  After warm up is also when the idle speed is adjusted via the big idle screw.  Book idle value is 1100 RPM.  Note that when the engine is cold, the idle stop will be at an RPM too low for the engine to run, and must be kept up with throttle grip.  After warm up, the engine will again idle on it's own at 1100 RPM.

The 022a carbs have slide adjusters for each slide position. The drill bit method frequently gets people in trouble, as it allows all the adjusters to be skewed where the slide can't physically close regardless of idle knob position.  While the carbs are off, make sure at least one carb can reach all the way to the carb "floor".  Lock that adjustment off, and then use that carb as a master carb to adjust the others to equal.
This applies to both bench and vacuum sync.

Ignore the numbers on vacuum sync gauges.  Do ensure the manometers can read equal from a common source.  Then adjust each carb to equal the reading on the master carb, and you are done with the sync.  You must have all the choke butterflies open = choke off, as those butterfly plates can effect what each carb allows for vacuum reading.

Do you still have the original pilot jets (#38)?  Did you examine the replacement jet emulsion tubes with respect to hole size, number of holes, tube length, and placement along the tube?
If the pilot jet is non-stock, you may need a different air screw setting.  I usually set this for throttle response under load for the lowest setting that gives me good engine pick up while in any gear, including 5th, with a throttle input of 1/2 total throttle travel.  This makes the idle mixture over rich, but, not enough to create a fouling build up on the spark plugs.

FYI, I had a no name 4 into 2 system on my 74 CB550 for 15 years before a swapped back to the 4 into 4 system.  No carb changes were ever needed for that exhaust change.

I would never change the main jet unless a plug deposit reading or a dyno fuel map report told me that was necessary.

Cheers,

[lucky]

I don't believe there is any need for larger jets.  There is certainly not enough info so far to make such a determination.
And, the present configuration of the bike is such a minor deviation from stock that jetting changes are unwarranted, imo.

Certainly clean the emulsion tubes.

Choke terminology:
Choke butterflies closed, equals choke "ON".  For the 022A carbs, (which is what a 1975 Cb550K model would have had leaving the showroom) choke on is lever up position for this condition.

The choke increases throat pressure differential, which forces more fuel from all jets while blocking air intake.
After warm up, no choke should be need, and the lever should be fully down.  After warm up is also when the idle speed is adjusted via the big idle screw.  Book idle value is 1100 RPM.  Note that when the engine is cold, the idle stop will be at an RPM too low for the engine to run, and must be kept up with throttle grip.  After warm up, the engine will again idle on it's own at 1100 RPM.

The 022a carbs have slide adjusters for each slide position. The drill bit method frequently gets people in trouble, as it allows all the adjusters to be skewed where the slide can't physically close regardless of idle knob position.  While the carbs are off, make sure at least one carb can reach all the way to the carb "floor".  Lock that adjustment off, and then use that carb as a master carb to adjust the others to equal.
This applies to both bench and vacuum sync.

Ignore the numbers on vacuum sync gauges.  Do ensure the manometers can read equal from a common source.  Then adjust each carb to equal the reading on the master carb, and you are done with the sync.  You must have all the choke butterflies open = choke off, as those butterfly plates can effect what each carb allows for vacuum reading.

Do you still have the original pilot jets (#38)?  Did you examine the replacement jet emulsion tubes with respect to hole size, number of holes, tube length, and placement along the tube?
If the pilot jet is non-stock, you may need a different air screw setting.  I usually set this for throttle response under load for the lowest setting that gives me good engine pick up while in any gear, including 5th, with a throttle input of 1/2 total throttle travel.  This makes the idle mixture over rich, but, not enough to create a fouling build up on the spark plugs.

FYI, I had a no name 4 into 2 system on my 74 CB550 for 15 years before a swapped back to the 4 into 4 system.  No carb changes were ever needed for that exhaust change.

I would never change the main jet unless a plug deposit reading or a dyno fuel map report told me that was necessary.

Cheers,

I basically agree.
A mixture adjustment may do the trick on this set up, but we shall see.

[fiolola]

Agree with TT. The drill bit fiasco drove me insane, after losing my hair looking for something thinner than a drill bit I came across a guitar E-string #8. After using the E-string as a gauge to bench synch the carburetors the bike came alive. gremlins removed. It has two into one factory headers with home made mufflers and a modified air box.

[Cut]

ALCON,

As a rookie noob, I was trying to dial in the carbs with the choke completely open.  Thanks to you "old timers" for clueing me in and taking it easy on me this go round.

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[phil71]

Wait, what?