Author Topic: Sort o rich, sort of lean?  (Read 2693 times)

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Offline brett_bike

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Sort o rich, sort of lean?
« on: September 27, 2014, 08:42:09 PM »
'73 CB500 been riding and chasing a stumble over the years. Project has been on the back burner since the birth of my kids, but I'm back at it. Finally got rid of the stubble off the line, but it still stumbles a bit when at cruising speed, but that's not the reason for this thread. Since I've been ridin again I've seen something I can't figure out.

Number one cylinder spark plug reads a little lean (white with a brown ring near the tip), and the other three plug are dry sooty black. I've been inside these carbs a countless number of times and all seems to be clean and clear. I'll get some pics posted tomorrow.

Other than the blubbering at cruising speeds it pulls strong ad rides great.  If I'm accelerating it ramps up great, but when cruising, as soon as I give it has it kind of blubbers before it picks back up.

Obviously I'm missing something so I'm hoping the wealth of knowledge compiled in this forum can help.

The bike is a 1973 CB500 with stock air box, 627 carbs with stock internals and settings.  The only non stock part is the MAC exhaust (which I don't care for but I needd something). 

Offline brett_bike

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Re: Sort o rich, sort of lean?
« Reply #1 on: October 05, 2014, 10:53:14 AM »
FWIW, here are what my plug deposits look like. Left to right, 1-4.

Strange that 1 & 3 would look good, while 2 & 4 are rich.

Offline HondaMan

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Re: Sort o rich, sort of lean?
« Reply #2 on: October 05, 2014, 11:42:03 AM »
Have you tightened the hose clamps, and replaced the little O-rings in the castings that hold the carbs to the head? These can have very small leaks that can't be easily detected. Unlike cars, when these have vacuum leaks, they run rich on that cylinder where the leak is, and lean on the previous one in the firing order, when they leak. If more than one leaks, all plugs tend to get sooty and black.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

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Offline TwoTired

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Re: Sort o rich, sort of lean?
« Reply #3 on: October 05, 2014, 11:42:30 AM »
I see no mention of a vacuum sync. or the standard 3K tune up being performed, including tappet lash.

Also:
No mention of float height setting or use of any aftermarket parts in the carbs.  How do you know the carb internal parts are stock?  Or, that they weren't altered by a P.O?
Emulsion tube cleaning was non mentioned and is often overlooked by those unfamiliar with the 627B, particularly with the mains.
Pilot screw setting unknown.  In fact, neither is the Jet needle position,  (if we assume all you've done was perfect, we will be in the same quandary are you, won't we?)

Age and condition of air filter?

Didi you notice any corrosion inside the carbs, particularly around the main jet towers?
Are all the seals new?
Lloyd... (SOHC4 #11 Original Mail List)
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Offline brett_bike

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Re: Sort o rich, sort of lean?
« Reply #4 on: October 05, 2014, 12:48:32 PM »
Hondaman, I picked up the O rings you're speaking about and plan to install them this evening.  Which hose clamps?

TT, I had a feeling you'd chime in with those questions (do you copy and paste that from a spreadsheet or actually type it out every time?).

Completely stock 627b carb's.
Just had the cabs off again to check fuel level and they're all 22mm.
40 primary
100 mains with new Honda seals
original needle (don't know number off hand) and emulsion tubes proven clean and clear and needle clip on #3.
All original Honda parts.
Unfortunately I'm running MAC exhaust and I'm in Denver at 5280 feet above sea level.
Paper filter, maybe dirtier than I'd like, but I just moved and can't find the uni replacement I purchased last year. I do have a k&n filter that mounts to the stock air plenum, but haven't tried that.
Towers have a bit of corrosion but the mains press in and stay put.

Thanks for chiming in Hondaman & TwoTired

Offline TwoTired

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Re: Sort o rich, sort of lean?
« Reply #5 on: October 05, 2014, 05:28:24 PM »
A dirty air filter will make the carbs provide rich mixtures.  In any event, you do NOT want to fine tune carbs using an other than pristine air filter.

The o rings on the mains are not there to provide retention.  They are there to make a seal, allowing fuel to only pass through the main jet.

Still no mention of a carb sync.

You were asking why some plug looked different than others.   I addressed possibilities in my prior post and you only read what you anticipated.  I typed that out earlier today, fyi.

Good luck!
Lloyd... (SOHC4 #11 Original Mail List)
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Offline HondaMan

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Re: Sort o rich, sort of lean?
« Reply #6 on: October 05, 2014, 10:57:17 PM »
Hondaman, I picked up the O rings you're speaking about and plan to install them this evening.  Which hose clamps?

TT, I had a feeling you'd chime in with those questions (do you copy and paste that from a spreadsheet or actually type it out every time?).

Completely stock 627b carb's.
Just had the cabs off again to check fuel level and they're all 22mm.
40 primary
100 mains with new Honda seals
original needle (don't know number off hand) and emulsion tubes proven clean and clear and needle clip on #3.
All original Honda parts.
Unfortunately I'm running MAC exhaust and I'm in Denver at 5280 feet above sea level.
Paper filter, maybe dirtier than I'd like, but I just moved and can't find the uni replacement I purchased last year. I do have a k&n filter that mounts to the stock air plenum, but haven't tried that.
Towers have a bit of corrosion but the mains press in and stay put.

Thanks for chiming in Hondaman & TwoTired

The hose clamps are the ones holding the rubber hoses to the castings, and to the carbs. The ones nearest the engine can get loose because the heat shrinks the hoses over the years. Often, the neoprene itself shrinks from the fuel exposure, too, especially with ethanol. I've been switching to Oetiker clamps lately: they seal much easier and with FAR less pressure-force on the rubber, preventing cracks in the future. They actually cost less than Honda wants for their clamps now, but they are not so cosmetically pretty.

I've also had quite some troubles with the tiny O-rings on the mainjets, lately. They are 1x4 size, but the ethanol swells them when wet, and they tend to leak slightly around the seal between the jet and the O-ring. You'll know: when you go to put the jet in, the dang thing won't stay if you hold the carb upright: it just lets the clip & jets fall right out. I haven't found some Viton 1x4 yet, still looking. The common Buna versions last a year or two before this crops up.

If you can fit that K&N in place of the foam filter, you'll be WAY ahead: we just demo'ed that on a 500 "chopper" last summer. I just got an old paper 500 filter and a large K&N filter: I hope to cut the K&N gauze from their filter and fit it to the paper one in lieu of the paper. Then I'm going to do it to my 350F, too! It will make a perfect sub.

All I need to finish this project is to find some of the gas-proof 2-part silicone sealant used to glue the screens on the K&N. There is a guy in Englewood who makes special K&N-like filters for snowmobilers in his garage for all the high-country rental companies: I once reprogrammed his PLC to quit spitting the goo all over when he wasn't using it. He still owes me a squirt or two! :)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline brett_bike

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Re: Sort o rich, sort of lean?
« Reply #7 on: October 06, 2014, 11:23:33 AM »
Thanks for the reply fellas.

TT, I sync' the carbs again last night with the Morgan carb tune tool. Was pretty much spot on. The air filter looks pretty dirty so I'm going to clean and install the K&N filter and see if that helps.  Also, the valves are adjusted to spec.

Hondaman,  maybe it's time to replace the main jet O rings, it's been a few years since I've done that.

I'll try installing the K&N filter and do another plug chop and repost my readings.

Offline harisuluv

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Re: Sort o rich, sort of lean?
« Reply #8 on: October 06, 2014, 11:43:47 AM »
Not something necessarily to look at first thing, but I want to give some input on issues that I have followed up on.  Out of all the models I work on, the CB500 in particular seems to have issues with running rich.  I always will try to answer any questions on tuning or any issues after a rebuild I do, and on the other carbs, it's almost never an issue.  However, with the cb500 I have had several cases where it just runs rich after trying just about everything.  I once was able to send just another set of 4 needles and that cured the problem for someone.  Most recently, a gentleman acquired OEM needle jet and emulsion tube sets (4) and that fixed his rich condition. 

I'm not sure exactly what it is, maybe the dimensions are just less forgiving to a lesser amount of wear, something about the metallurgy of that particular brass, etc.  However, I have come across this enough and noticed that it's almost all with the CB500 to say it's an "issue" with 627B carbs.  Just something to keep in mind.

Offline Deltarider

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Re: Sort o rich, sort of lean?
« Reply #9 on: October 06, 2014, 12:28:28 PM »
Quote
Not something necessarily to look at first thing, but I want to give some input on issues that I have followed up on.  Out of all the models I work on, the CB500 in particular seems to have issues with running rich.  I always will try to answer any questions on tuning or any issues after a rebuild I do, and on the other carbs, it's almost never an issue.  However, with the cb500 I have had several cases where it just runs rich after trying just about everything.  I once was able to send just another set of 4 needles and that cured the problem for someone.  Most recently, a gentleman acquired OEM needle jet and emulsion tube sets (4) and that fixed his rich condition. 

I'm not sure exactly what it is, maybe the dimensions are just less forgiving to a lesser amount of wear, something about the metallurgy of that particular brass, etc.  However, I have come across this enough and noticed that it's almost all with the CB500 to say it's an "issue" with 627B carbs.  Just something to keep in mind.

Could well be but note that in other markets (like Europe) needle clips were in 3rd groove (one notch leaner) and not in 4th as in US, that is if we are to believe that American Honda spec booklet.
My estimation is that the needle in 4th combined with a #100 main jet must lead to a rich condition. Very rich indeed.
« Last Edit: October 06, 2014, 12:46:12 PM by Deltarider »
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Offline harisuluv

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Re: Sort o rich, sort of lean?
« Reply #10 on: October 06, 2014, 01:34:16 PM »
Good point!  I am aware of this though, and my observations are based on the corresponding proper needle position for the carb/market and many of these scenarios I am speaking of involve even going from 4-3 with no improvement.

I'm not claiming any relation to OP's situation, I'm just putting forth the issue is possible.  It wouldn't be my first inclination from what I have seen in this case.
« Last Edit: October 06, 2014, 01:42:41 PM by harisuluv »

Offline Rocky2010

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Re: Sort o rich, sort of lean?
« Reply #11 on: October 07, 2014, 04:48:18 PM »
FWIW, here are what my plug deposits look like. Left to right, 1-4.

Strange that 1 & 3 would look good, while 2 & 4 are rich.

Looking at your plugs my CB500 does the same thing, it runs ok but I would like to get the plugs to be the same.
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Offline HondaMan

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Re: Sort o rich, sort of lean?
« Reply #12 on: October 08, 2014, 04:18:25 PM »
Good point!  I am aware of this though, and my observations are based on the corresponding proper needle position for the carb/market and many of these scenarios I am speaking of involve even going from 4-3 with no improvement.

I'm not claiming any relation to OP's situation, I'm just putting forth the issue is possible.  It wouldn't be my first inclination from what I have seen in this case.

I do remember the early 1971 CB500 being recalled to have the needle JET replaced in the carbs: these bikes also lost their "HONDA" plate off the alternator when the short pins let it fall off. I didn't get any of these in my own shop, and my own 500 was past the recall serial number, and neither I nor my brother's 500 had a problem. This makes me wonder about the needles?

Harrisluv: have you noticed wear marks on these needles?

For reference: the needle notch in the 627 carbs makes about 1.5% change in jetting for the given throttle opening. In the CB750 pre-1976 carbs (all) it is almost as much.

Also on the CB750 high-mileage engines: the hole in the Needle Jet eventually becomes wider (egg-shaped) toward the engine from the needle being pulled gently against it by vacuum. This makes them rich in the low midrange, which, if "corrected" by installing a leaner mainjet, then makes it fall on its face above 1/2 throttle for too-lean mix. Food for (500) thought?
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

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Offline Deltarider

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Re: Sort o rich, sort of lean?
« Reply #13 on: October 09, 2014, 08:23:23 AM »
Quote
I do remember the early 1971 CB500 being recalled to have the needle JET replaced in the carbs
That's intriguing. I had a look in the Parts List and indeed in the oldest Parts List you'll find jet and needle listed separately each with its own number.
cf. http://www.honda4fun.com/pdf_parts_list/pdf_spac500/CB500-71/CB500-71_2.pdf
In the Parts Lists for later 500 models jet and needle are a set with one number.
Quote
: these bikes also lost their "HONDA" plate off the alternator when the short pins let it fall off. I didn't get any of these in my own shop, and my own 500 was past the recall serial number, and neither I nor my brother's 500 had a problem.
Aahhh... mystery solved. I'll explain: plate and pins have the same parts numbers so no modifications BUT... on later models most probably Honda used some sort of glue as an extra. That would explain the three discoloured spots close to the holes where the pins sit. I always wondered why these spots were there. BTW I've nevertheless lost my Honda plate, be it after 110.000 kms.

Quote
For reference: the needle notch in the 627 carbs makes about 1.5% change in jetting for the given throttle opening.
Is that 1.5% CO?


« Last Edit: October 09, 2014, 08:29:22 AM by Deltarider »
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Offline harisuluv

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Re: Sort o rich, sort of lean?
« Reply #14 on: October 09, 2014, 11:48:10 AM »
Good point!  I am aware of this though, and my observations are based on the corresponding proper needle position for the carb/market and many of these scenarios I am speaking of involve even going from 4-3 with no improvement.

I'm not claiming any relation to OP's situation, I'm just putting forth the issue is possible.  It wouldn't be my first inclination from what I have seen in this case.

I do remember the early 1971 CB500 being recalled to have the needle JET replaced in the carbs: these bikes also lost their "HONDA" plate off the alternator when the short pins let it fall off. I didn't get any of these in my own shop, and my own 500 was past the recall serial number, and neither I nor my brother's 500 had a problem. This makes me wonder about the needles?

Harrisluv: have you noticed wear marks on these needles?

For reference: the needle notch in the 627 carbs makes about 1.5% change in jetting for the given throttle opening. In the CB750 pre-1976 carbs (all) it is almost as much.

Also on the CB750 high-mileage engines: the hole in the Needle Jet eventually becomes wider (egg-shaped) toward the engine from the needle being pulled gently against it by vacuum. This makes them rich in the low midrange, which, if "corrected" by installing a leaner mainjet, then makes it fall on its face above 1/2 throttle for too-lean mix. Food for (500) thought?

Ok that explains a lot.  Good to know.  Yes the needles don't show any "wear" really.  They have very clear horizontal machining marks on them that are still there.  Also when they were replaced with new OEM replacements, person looked at the needles also and say they looked perfectly fine.

Offline Deltarider

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Re: Sort o rich, sort of lean?
« Reply #15 on: October 09, 2014, 11:59:52 AM »
I remember a mechanic (some 30 years ago) saying they'd hardly seen wear problems with CB500 carbs brass parts in their workshop much in contrast with Goldwing 'brass'.
« Last Edit: October 09, 2014, 12:04:17 PM by Deltarider »
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Offline harisuluv

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Re: Sort o rich, sort of lean?
« Reply #16 on: October 09, 2014, 12:46:10 PM »
I remember a mechanic (some 30 years ago) saying they'd hardly seen wear problems with CB500 carbs brass parts in their workshop much in contrast with Goldwing 'brass'.

Guess that explains the recall!

Offline HondaMan

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Re: Sort o rich, sort of lean?
« Reply #17 on: October 09, 2014, 08:33:04 PM »
Quote
I do remember the early 1971 CB500 being recalled to have the needle JET replaced in the carbs
That's intriguing. I had a look in the Parts List and indeed in the oldest Parts List you'll find jet and needle listed separately each with its own number.
cf. http://www.honda4fun.com/pdf_parts_list/pdf_spac500/CB500-71/CB500-71_2.pdf
In the Parts Lists for later 500 models jet and needle are a set with one number.
Quote
: these bikes also lost their "HONDA" plate off the alternator when the short pins let it fall off. I didn't get any of these in my own shop, and my own 500 was past the recall serial number, and neither I nor my brother's 500 had a problem.
Aahhh... mystery solved. I'll explain: plate and pins have the same parts numbers so no modifications BUT... on later models most probably Honda used some sort of glue as an extra. That would explain the three discoloured spots close to the holes where the pins sit. I always wondered why these spots were there. BTW I've nevertheless lost my Honda plate, be it after 110.000 kms.

Quote
For reference: the needle notch in the 627 carbs makes about 1.5% change in jetting for the given throttle opening.
Is that 1.5% CO?




Well, it's 1.5% richness: if the engine is burning it all OK, it won't show up at all. If the mix is too lean, then the spark won't ignite it, and it shows up in the pipes as CO (unburned fuel).

BTW: this is why some are confused about the air screws, especially when they have 4-1 pipes. (It also appears somewhat in 4-4 pipes.) If the 4 cylinders are running reasonably well and one air screw is turned in to the point where the mix becomes too lean, that cylinder stops burning most of the charge. This shows up as excess CO in the pipes, because it becomes unburned fuel. If the same screw is slowly backed out the CO goes down, but if you continue to back it out, it goes back up again when the mixture becomes so rich the cylinder can't burn it at the speed the engine is running. This is why we only have about 3/4 turn useful range on these air screws. The 750 carbs 1969-1976 only have 1/2 turn range.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

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Re: Sort o rich, sort of lean?
« Reply #18 on: October 10, 2014, 01:32:44 PM »
Thanks for explaining the bit about the effect of mixture screws and CO, Hondaman. I've been thinking about this since reading your book!

Offline brett_bike

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Re: Sort o rich, sort of lean?
« Reply #19 on: October 13, 2014, 08:18:23 PM »
We'll, it looks like I'll be pulling the carbs again and see if I can find replacement needles, and install the O-rings that Hondaman was talking about. Holy hell, in the 8 years I've owned this bike this will be 20'somethingth time I've had the carbs off chasing one thing or another. Quite the learning experience.

Offline TwoTired

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Re: Sort o rich, sort of lean?
« Reply #20 on: October 14, 2014, 10:34:42 AM »

BTW: this is why some are confused about the air screws, especially when they have 4-1 pipes. (It also appears somewhat in 4-4 pipes.) If the 4 cylinders are running reasonably well and one air screw is turned in to the point where the mix becomes too lean, that cylinder stops burning most of the charge. This shows up as excess CO in the pipes, because it becomes unburned fuel. If the same screw is slowly backed out the CO goes down, but if you continue to back it out, it goes back up again when the mixture becomes so rich the cylinder can't burn it at the speed the engine is running. This is why we only have about 3/4 turn useful range on these air screws. The 750 carbs 1969-1976 only have 1/2 turn range.

So, is this a supportive argument for the purported air screw behavior description, of turning in = leaner and out = richer?  ...That we are fooling ourselves with an exhaust gas analyzer?

Perhaps you can also educate us why the spark deposits reduce when the air screws are turned out, and they blacken when turned inward.  There is also the engine response behavior to consider.  Turn air screws outward and the engine stumbles (wheezes) on advance, and recovers cleanly when the the throttle is returned.  Turn the air screws inward and the throttle response improves but the plugs accumulate black carbon deposits, much like when adding choke (enrichment).

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KiefRichards

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Re: Sort o rich, sort of lean?
« Reply #21 on: October 14, 2014, 11:01:25 AM »

BTW: this is why some are confused about the air screws, especially when they have 4-1 pipes. (It also appears somewhat in 4-4 pipes.) If the 4 cylinders are running reasonably well and one air screw is turned in to the point where the mix becomes too lean, that cylinder stops burning most of the charge. This shows up as excess CO in the pipes, because it becomes unburned fuel. If the same screw is slowly backed out the CO goes down, but if you continue to back it out, it goes back up again when the mixture becomes so rich the cylinder can't burn it at the speed the engine is running. This is why we only have about 3/4 turn useful range on these air screws. The 750 carbs 1969-1976 only have 1/2 turn range.

So, is this a supportive argument for the purported air screw behavior description, of turning in = leaner and out = richer?  ...That we are fooling ourselves with an exhaust gas analyzer?

Perhaps you can also educate us why the spark deposits reduce when the air screws are turned out, and they blacken when turned inward.  There is also the engine response behavior to consider.  Turn air screws outward and the engine stumbles (wheezes) on advance, and recovers cleanly when the the throttle is returned.  Turn the air screws inward and the throttle response improves but the plugs accumulate black carbon deposits, much like when adding choke (enrichment).


I really wish I had paid more attention to the O2 values when did this years ago, but I didn't fully understand back then...
Do you remember TwoTired?  If screwing it in too far causes a lean misfire, 02 values should be WAY high.

Offline TwoTired

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Re: Sort o rich, sort of lean?
« Reply #22 on: October 14, 2014, 01:56:34 PM »
I really wish I had paid more attention to the O2 values when did this years ago, but I didn't fully understand back then...
Do you remember TwoTired?  If screwing it in too far causes a lean misfire, 02 values should be WAY high.

In my experience, screwing it inwards too far never caused a lean misfire (hollow tipped air screws), until the throttle was opened, slides raised.  Then the already lean mixture drifted out of the combustion zone, the engine simply wheezed and produced no power until the slides were then lowered.  The non-accelerator pump carbs need the idle mixture to be on the rich side, so that when the slides are raised the vacuum drop simultaneously reduces carb throat vacuum (the pull on the fuel jets, AND allows higher volumes of air to pass through the carbs.  It's why you can't whack the throttle full open at low RPM, particularly under load.  The hollow tipped screws always allowed some air bypass and prevented total shut off the the pilot emulsion process.  During 70's development the EPA wanted better control/limitations over idle mixtures in the pursuit of cleaner exhaust emissions.  As I see it, they first tried to placate the bureaucrats by making the air screw tips solid, making the control more effective at influencing the exhaust sniffer.

My analyzer is indeed a % CO indicator.  No 02 sampling.  But, I am NOT talking about taking an individual carb's pilot screw to extremes, neglecting the other three.  But rather, turning them all inward (or outward) in the same increments and then monitoring the effect at the exhaust, the spark plug deposits, and the engine operational response behavior.  I don't tune to shut off any of the cylinders, which might allow a cylinder to pump out raw inlet mixtures.  In my experiment, I turned them all while sniffing just the one (of 4) exhausts.   At 5-6 turns out, there wasn't much adjustment effect, and checking each of the exhausts, there was very little difference among them and any more tweaking was ineffectual.  The engine ran wonderfully on the center stand, with hardly any plug deposits, but would pick up poorly when throttle was given.  This became far worse when any sort of load was placed on the engine, making it un-rideable.  Turning in the air screws all four at a time (to enrich the idle mixture) in increments gradually improved the throttle response.  when it finally became street driveable, I check the turns count of the air screws, and it was right where the book stated to set them on a stock bike.  I then started turnig them inwards (all four) and the throttle response got even better regarding how much twist I could give it in top gear from idle.  To a point, as these air screws had that hollow tip air screw bypass.  But, the spark plugs were collecting some serious soot deposits.  They would only self clean with some high RPM drag style runs, to burn off the soot.  Any prolonged idling and, god forbid choke, and the plugs would eventually foul.
The bike used was 74 CB550 K0, with 022a carbs.  So, there could be an argument that 750 Carbs behave differently.  However, no one has pointed out a pilot circuit design difference between the CB550 And the CB750 of that era.  Indeed all the internal arrangements and passageways appear the same, apart from the obvious different casting mold.

Beware solid tipped screws can shut the air jet supply off completely, then there is no emulsion, and the throat vacuum pulls in only raw fuel from the pilot jet.  I suspect this is why Keihin evolved the carbs to an IMS arrangement, as in the PD style carbs.  These regulate the emulsified fuel final volume just as it enters the carb throat.  With the PDs on the CB750, this allowed very lean mixtures during idle.  It is also the reason why accelerator pumps were added to deal with the sudden slide opening from the mechanically linked twist grip.
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.