Topic: Air Srew Adj. ?????????

[DYSKORD]

I know, I know . . . I have read all posts containing the terms "pilot" and "air screw" and "adjustment" and I am still wondering a few things:

1. Initial setiings for 76 CB750F is 1and3/8 out (per "f" supplement in shop manual), right? Seems like alot. Smells like alot (rich)!???
2. Do you adjust all four carbs the same incremental turns to reach max idle or adjust one to get a max idle then move on to another and adjust to max idle then on to next and the next?
3. Does it matter which carb you start with?
4. What tach would be recommended for the adjustment? If needed at all. Mine (cyclex mini tach), doesnt seem to move at all with previous adjustment attempts.

I have done an initial carb sync and will recheck after mixture adjustment.
Thanks for the help guys.

[Spanner 1]

That max. idle thing is hooey !.... unless your carbs are 'pristine' and your ignition and all adjustments, sync. etc. spot-on, the effect of slow jet mix adjustment is 0 effect on rpms IMO. Very hard to see any difference in mixture vs. rpms........ that's been my experience anyway  ... 1 turn on your 069A carbs is the norm.

[Skunk Stripe]

To use the rpm measurement, you need a sensitive tach. The bike tach or cyclex will not do it. You need something that will register as low as 30 rpm.
What you are supposed to do is tune one carb to max rpm and then back it down 100 or something like that, then repeat on each carb.
You cannot do this by ear. A tach such as one used with a dwell meter would probably work.
I would start with carb 2 as it is the baseline carb for syncing. That is my only reasoning, it may not make any difference.

[DYSKORD]

To use the rpm measurement, you need a sensitive tach. The bike tach or cyclex will not do it. You need something that will register as low as 30 rpm.
What you are supposed to do is tune one carb to max rpm and then back it down 100 or something like that, then repeat on each carb.
You cannot do this by ear. A tach such as one used with a dwell meter would probably work.
I would start with carb 2 as it is the baseline carb for syncing. That is my only reasoning, it may not make any difference.

You can do it by ear, you just have to listen to the bike, it will tell you whats going on.

So if a stock tach isnt accurate enough then I woudnt think my ear is either.

[Skunk Stripe]

I seriously doubt you can. Sure you MIGHT hear a difference but it will be nowhere as accurate as a decent tach. It just is not possible, especially if you have more than 1 pipe. You can't tell the sound from 1 pipe from another.

[DYSKORD]

I seriously doubt you can. Sure you MIGHT hear a difference but it will be nowhere as accurate as a decent tach. It just is not possible, especially if you have more than 1 pipe. You can't tell the sound from 1 pipe from another.

Exactly
4-1 here.

[TwoTired]

If you have carbs with accelerator pumps, you can set the pilot screws for "max lean" which would be the fastest or peak idle speed.   The PD carbs can be set with a very sensitive tach.   But, all the screws should be pretty close to equal setting among them.

Carbs that do not have accelerator pumps, and an air bleed pilot screw are not set with a tach.  They are set best response to throttle twist.
These carbs must run rich at idle because when the throttle is suddenly opened the carb throat vacuum is vastly reduced, which make the jets stop or greatly reduce their fuel flow. This, at the same time more air is allowed to pass the carb's venturi.  To get the engine to pick up, the idle setting has to be over rich so the engine will pick up.  All the carbs (and cylinders) should behave the same, so the pilot screw setting should be the same for all carbs.  Faults elsewhere should not be "covered up" with unequal pilot screw settings.

Anyway, for non-PD/non-accelerator pump carbs, the idle mix is set just lean enough so the that the plugs don't foul during idle, but the engine will pick up when the throttle is twisted to one half of total travel and the engine will still pick up with smooth power from idle speed in any gear, which is an over rich setting.

Usually, the carbs are set to factory position to achieve this.  The installation of pods or other induction mods change the carb throat pressures and stock settings then rarely apply.  You have to find new settings to accommodate the induction changes.

Also, if the needle jet isn't sized or adjusted properly it can leak a lot more fuel into the carb throat than it should at idle throttle position.  After market needles that don't conform to stock profile can also mess up idle mixture, making the air screw adjustment, er, "difficult".

Cheers,

[DYSKORD]

If you have carbs with accelerator pumps, you can set the pilot screws for "max lean" which would be the fastest or peak idle speed.   The PD carbs can be set with a very sensitive tach.   But, all the screws should be pretty close to equal setting among them.

Carbs that do not have accelerator pumps, and an air bleed pilot screw are not set with a tach.  They are set best response to throttle twist.
These carbs must run rich at idle because when the throttle is suddenly opened the carb throat vacuum is vastly reduced, which make the jets stop or greatly reduce their fuel flow. This, at the same time more air is allowed to pass the carb's venturi.  To get the engine to pick up, the idle setting has to be over rich so the engine will pick up.  All the carbs (and cylinders) should behave the same, so the pilot screw setting should be the same for all carbs.  Faults elsewhere should not be "covered up" with unequal pilot screw settings.

Anyway, for non-PD/non-accelerator pump carbs, the idle mix is set just lean enough so the that the plugs don't foul during idle, but the engine will pick up when the throttle is twisted to one half of total travel and the engine will still pick up with smooth power from idle speed in any gear, which is an over rich setting.

Usually, the carbs are set to factory position to achieve this.  The installation of pods or other induction mods change the carb throat pressures and stock settings then rarely apply.  You have to find new settings to accommodate the induction changes.

Also, if the needle jet isn't sized or adjusted properly it can leak a lot more fuel into the carb throat than it should at idle throttle position.  After market needles that don't conform to stock profile can also mess up idle mixture, making the air screw adjustment, er, "difficult".

Cheers,

I was wondering where you were
I have about 2k on a fresh topend +1mm. Pods. 4-1. 115 mains. Power Arc ignition. All carbs (not PD) are pulling about 20cm +/-2 on the vac guages. If you dont set it based on rpm, you set it to throttle response? So does 1 and 3/8 out from seated sound like a good starting point. I have tried 1 full turn per k model spec and the previously mentioned F spec. Idles better with K spec but dont have any ride times to verify other aspects of performance. Should I start with K initialy or F preliminary settings? What is the process for determining the correct mixture if highest rpm isn't the best? Idle plug chops?

[TwoTired]

I set the 550 Carbs (same technology), using how the throttle responds to sudden opening.
What you can try is opening them up 3 or 4 turns.  (Yeah, probably too much but you will learn something; read on.)  Mark the throttle positions on the bars so you know where one half of total travel is.  Go ride the bike, and "Cruise" in top gear at idle RPM.  Whack the throttle to the one half position.  The bike should pick up smoothly/predictably (albeit slowly) and go faster.  If it wheezes and falls flat until you return the throttle back to idle, then screw all the idle screws in 1/8 or 1/4 turn each.  Repeat the above test.  It should be a bit better as to how much (or far) you can twist and still get smooth pick up.  Keep making the turn-in adjustments until you can whack the throttle to the one half position from idle "cruise", and the bike picks up smooth and predictable.  That's as lean a setting the idle should be to make a nice street machine, where you can rely on throttle response when needed in any gear the bike is in.

You can keep turning the screws in until it responds with more throttle travel, say 3/4 twist.  But, that will really be a too rich setting where prolonged idle with begin to soot up the plugs.  Sooted plugs will sputter/blubber on pickup, rather than wheeze or fall on its face with zero power.

That's how I make the adjustment.

Cheers,

[scottly]

I seriously doubt you can. Sure you MIGHT hear a difference but it will be nowhere as accurate as a decent tach. It just is not possible, especially if you have more than 1 pipe. You can't tell the sound from 1 pipe from another.

Exactly
4-1 here.

If you can tune a guitar with 6 strings by ear, you can tune the idle of a four cylinder motor by ear, too. The human ear is very sensitive to shifts in frequency, (RPM) at these ranges...

[Spanner 1]

The sad thing is you can turn the air screw in-and-out all day long on those 069A carbs and nary a change in beat will be heard........ set it at 1 turn out and forget it.... be much more involved with your mid/ WOT response with your non stock set-up... and your overall air/fuel mix across the range... that's if you care to ride your bike across a bigger circle than about 20 miles ( some blokes don't ever, I know, and that's O.K. )
No diss intended, btw.

[DYSKORD]

Thanks TwoTired, that makes more sense. I will try your technique from stationary first and get her in the ballpark, then hone it in when it warms up this weekend. I will give it a shot.
  

The sad thing is you can turn the air screw in-and-out all day long on those 069A carbs and nary a change in beat will be heard........ set it at 1 turn out and forget it.... be much more involved with your mid/ WOT response with your non stock set-up... and your overall air/fuel mix across the range... that's if you care to ride your bike across a bigger circle than about 20 miles ( some blokes don't ever, I know, and that's O.K. )
No diss intended, btw.

Spanner:
I thought throttle was either wide open or closed
Yeah it does see alot of 20miler intown riding but not shy of any 200+ milers on the weekends. She doesnt get babied on the 20milers though.

Scottly:
For the record I can/could tune a guitar by ear, and its not the same. That may be my inexperience. A single 4barrel Holley by ear, no prob. but 4 singles I am not there yet. ecspecially with a 4-1 exhaust.

[Skunk Stripe]

If you can tune a guitar with 6 strings by ear, you can tune the idle of a four cylinder motor by ear, too. Wink The human ear is very sensitive to shifts in frequency, (RPM) at these ranges...

Just cause you can hear difference does not mean you can tune an air screw by it. Sorry but I can also tune by ear and it is not the same.
I bet if you were to try tuning by ear and then tuned by tach, you would see just how far off you really are.

[Kong]

I would not fixate on the one adjustment that makes the least amount of difference on how the bike runs, I'd just set it at 1 3/8 turn, or whatever the manual (as amended) dictates and move on to the adjustments that do make a difference.  Get the needle height right, set the slides, experiment with jets and float heights, how about those emulsion tube holes - size and location.

There is only one reason I say this.  It is the shape of the end of the screw(s) in question.  Notice that the end is square cut and not tapered?  That means that it is an On/Off switch.  If it had a tapered needle-like end then adjustments would change the flow of something, either air or fuel.  But our needles aren't tapered.  So that factory recommendation isn't something that you adjust around, its a number of turns necessary to open the valve, and the valve is designed to be open.  See what I'm saying?  If its screwed in too far its turned off, and that's wrong.  If its turned out too far it will fall out, and that's bad too.  Other than that all that matters is that it is loosened enough to be open, there is no actual adjustment capability to the thing.

On Edit:

I may have spoke out of turn here.  I was thinking of later year carburetors than yours and so I may be wrong about what kind of screw you have.  Yours may in fact be tapered on the ends - I don't know - and if they are tapered then they do have adjustment capability and you are doing good to seek advice.  So I'm sorry if I gave inappropriate advice above - but maybe not such a bad thing because it helps accent the difference.

[Skunk Stripe]

Actually on the later carbs, the idle mix screw makes a HUGE difference. The idle jets can affect your mix up to 3/4 throttle. Also on the later carbs, the screw IS tapered, not square.
To further add, 69-76, the screw is an air screw and out is leaner while in is richer.
77-78, it is a fuel screw so it meters fuel. On these screws, out is richer while in is leaner.

[Kong]

I am well aware that on some carbs the screws meter air and on some carbs the screws allow fuel to flow - or not, all I'm saying is that you are looking at two different types of valves that have different characteristics; things that are different are, well, different.   I say this as a general statement.  How the two different valves work is easy enough to understand and yes, even with the squared off screws and their matching seats - the ones that are essentially On/Off switches - there is of course some very very slight adjustability right around the opening point - but the valve was not designed to work as an adjusting mechanism other than the flow/no-flow options and whatever adjusting effect is at best minimal.  Tapered screws which fit into shaped seats are made that way so that the widest range of adjustability of whatever material flows through them is achieved consistent with the size of the orifice, that is just the nature of the design.  Just as a casual observation it seems to me that you see truly adjustable screws used when its air being metered and binary valves (+ or -, yes or no, On or off) where it is fuel that is being allowed - or not.

[Skunk Stripe]

And yet if you check posts years past, you will constantly be seeing long time members talking about adjusting the air or fuel screws and that it makes a difference.
I know for a fact on the 77/78, the screw does meter fuel and is tapered.
From my reading,On the earlier carbs, the screw does meter air. Any valve will meter, some are better than others. Even a basic gate valve can still meter, it just is not very accurate.
Regardless, any time you change the opening of a valve, it will change the amount that move through, be it air or water.

I think you may be the first one to say that the air screw does not matter.

[DYSKORD]

Kong keep the replies coming, I find I learn alot by litening to discussions/arguments/debates.
I have messed around with them and there seems to be a noticeable difference in the performance of the bike withh different settings. It may be variations somewhere else or placebo effect, but it "seems" necessary and adjustable. Its obvious the setting isnt necessary for the bike to run, but is, to run good. The problem is that I cant quite get a straightforward answer on procedure or starting point (except for TwoTireds, which is where I will startover until someone offers some other suggestion). You would of thought I had asked whats the best oil  .

[handsomeness]

just try running it into a curb...

that always fixes carbs in my experience

[DYSKORD]

just try running it into a curb...

that always fixes carbs in my experience

Yeah helps the mixture. Like those paint can shakers at Home Depot!

[Skunk Stripe]

D, I got a dwell meter a few years back that has a good tach on it, also useful for setting dwell on points which is more accurate than gap.
Get the bike warmed up good and hook up the tach. Just start with #1 and adjust till the tach shows the highest rpm and then back off just a bit, about 50-100 rpm or so. Follow the same step for each carb and you should be good.
Some have tried to use a thermal heat gun to set things but I am not sure how well that works as it is just a temp reading.
You can go by plug color but that is difficult to figure out. I know there is some OLD chart with pics floating around but that was made back in the 70s and gas has changed since then.
If you do not want to use a tach, then do plug chops. time consuming but it works. Start with a low speed chop, 2000rpm or so and run that for a bit, pull clutch and kill engine. Stop without releasing clutch. Check plug color.
Now this is open to debate but I feel that dark brown is too rich however I have the later carbs and they run lean. I would shoot for a medium to lightish tan color.

from there, a high speed chop with RPM above 7000 and run through the gears to keep load. On an empty stretch of road of course.
Pull clutch and kill same as low speed kill. Check color.
Again, going for about same color. once you have that, further tuning can be done with your needle height. Go back and forth in the rpm range and feel for dead or flat spots, adjust needle height to compensate.
once done, you may have to resync.
I have done this and it works adequately.

The very best way is to weld bungs into the pipes just at the bend going under the engine. Once at a tine, put in a wide band o2 sensor and the reader and adjust from there. Of course that is the expensive way to go.

[TwoTired]

Pretty hard to believe anyone would say the pilot screw setting doesn't matter.

All our SOHC4 carbs have pilot adjust screws.  The early type has air screws, which regulate air going to the pilot jet's emulsion tube.
The PD type has an IMS  (Idle Mixture Screw), which regulates the fuel flow after it has been mixed with air at the pilot jet exit.

The early type carbs often had a blunt, even hollow tip, but is still tapered and fits into a metering hole.  The blunt, hollow tip has a cross drilled hole, which prevents total closure of the air passage feeding the pilot's emulsion tube, even if the screw is fully seated. The 069a carbs of the CB550F had solid tipped, but tapered pilot screws.  I'm not certain of the early CB750 Carbs.  But, the design basics are equivalent among the SOHC4 mechanical slide examples.

The pilot system in both carb type serves to provide the minimum requirements of the engine to run at tickover.  It is hardly there for extraneous purposes.  I can't imagine why a manufacturer would pay for extra, un-needed parts into the design for a consumer product in a competitive marketplace, (this excludes government committee mandates, and maybe Harley traditions).

Cheers,