Is this right?

[deltarider]

Just read this in the international forum.
Question was: I just wanted to verify that turning IN (clockwise) the airscrews makes it run leaner. I ask only because I've been working under the assumption that turning them out brings in more air thus more lean. This is for a 1976 750F with 064A carbs.

The answer given is:
Yes.
The idle-air screw in those carbs (all of the 750 roundtop carbs) controls the amount of fuel-air mixture that gets pulled up for each intake stroke. It is already mixed as fuel & air inside the tiny tube on the top of the idle jet, where those holes are, when air from the little brass orifice at the back of the carb's bell met the rising fuel in the jet's metering hole. Then, the position of the screw admits more (or less) of this mixture during the intake suction.

I think the internet's description of these as "air screws" or "idle fuel screws" may be adding much to the confusion about these gizmos. It is the size of the jet (#40) that determines the RATIO of air-to-fuel, and the screw then controls HOW MUCH of this mixture gets added for each stroke. If you add too much, it runs rich, and vice-versa.

When the screw is turned more inward, it increases the amount of time required for the fuel-air mixture to rise up into the carb's throat. If turned more outward it lets this happen sooner during the intake stroke. In between each intake stroke the fuel falls back down to the little emulsifier holes in the top of those idle jets, and if the idle adjuster screw is turned inward more, it pinches off the flow, making things happen slower. This in turn makes the amount of [fuel + air] entering the engine less for that stroke, so ti runs leaner.

Years ago (notably in Honda's Shop Manuals for twins) Honda used the ambiguous translation phrase for the use of this screw that said, "For richness, turn screw in". It was this poor translation that seems to have started the confusion: in English this means, "If the engine is running rich, turn the screw inward". That adjustment reduces the amount of fuel/air mix being admitted, making it run leaner. This was an issue with the Honda Twins, as the vibration often made the screws slowly back out, causing richness and fouled sparkplugs that brought the bikes in for a tuneup. The CB350 twin was notorious for this.

How about the CB500/550s with the oldstyle carbs: 627B, 649A, 069A and 022A?

[K2-K6]

The twins in majority use CV type carbs (can't recall throttle slide example) that being the case, they operate differently and may cause confusion in using as a reference to understand the four cylinder engine and carburettor setting.

[Johnwebley]

The smaller twins CB200 era use slides, and operate the same as the 500,



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[Herr Supersport]

In general I believe every of his words.
But never understood his explanations to the airscrews.

Just to mention: some years ago I tested my carbs [CB400f] with colortunes. While idling turning the screw inwards changed the colour to orange/rich, turning out changed to bright-blue/lean.

Possibly an answer to 'is that right?'

[deltarider]

Long time ago I have used a Colortune. When I finally got the blue color, bike wouldn't accelerate anymore.
I know from experience which way airscrews should be turned. I don't understand Hondaman's view on the matter.
K2-K6 once gave here an on insight how the airscrews function and I found his explanation much better.

[Oddjob]

In richer, out leaner. Never heard anything different from Honda on any course, saying that it may change with CV carbs for instance. They tend to have the screws in different places on the carb body so may react differently.

[K2-K6]

Also same for me on these carbs fitted to sohc 4 engines, inwards makes it richer, out makes it leaner.

Didn't realise the author initially, thought before it's a strange view of it though.

There's a mix of metering method and setting the mixture kind of overlaying within a "conversation" there, which makes it seem convoluted. Wonder if anyone has asked or had communication as to why he thinks it's that way as written there.

Just checked manual for PD type and next generation CV carbs (early 80s) with neither stating which way turning the screw affects the mixture. They, both type,  have the adjustment screw on underside of the venturi engine side of carb, that's in comparison to the older slide type we are talking about here.

[Herr Supersport]

It's not my plan to bash somebody - on the contrary. But as a longtime-lurcher I probably can point to a link explaining "why he thinks it's that way as written there". Second post.

http://forums.sohc4.net/index.php/topic,182280.0.html

Wasn't that easy to understand for me, german-speaking guy. Even if I live in Bernoulli's town .

[JamesH]

In richer, out leaner. Never heard anything different from Honda on any course, saying that it may change with CV carbs for instance. They tend to have the screws in different places on the carb body so may react differently.

What Ken said...

[K2-K6]

It's not my plan to bash somebody - on the contrary. But as a longtime-lurcher I probably can point to a link explaining "why he thinks it's that way as written there". Second post.

http://forums.sohc4.net/index.php/topic,182280.0.html

Wasn't that easy to understand for me, german-speaking guy. Even if I live in Bernoulli's town .

Bernoulli...for anyone with a technical interest in F1 currently, the application of his work by RB technical team is quite impressive 

Thankyou for the link, I've read through there before and can understand why,  when translated or interpreted by a different language speaker, it seems confusing. It certainly wanders around through some very tenuous relationships used to demonstrate principles.

Some that caused me to question it when reading to myself, reference to Honda 250/6 1960s race engine missed the point of low rpm running completely, from my understanding of that bike.
I think I recall correctly that it had no idle circuit in carburation at all, revving it while stationary the only way to keep it running. It is a very radical engine though with approaching 300bhp per litre and still would be if produced now.
Principal among the need to rev it though is a crankshaft that has different sized main and big end bearings on outside cylinders, getting larger as more torque was added with each successive cylinder to take drive out from crank centre. Apparently so marginal in strength that you can twist the built crank assembly out of line by hand ! Riders had to keep above 14,000rpm in driving it to avoid doing the same to the crank. There was simply no need for any idle provision in carburation, it's not a good example to reference in my view.
Demonstrated by Steve Plater in recent times at IOM TT circuit, he stalled it twice from not concentrating on keeping it revving when slowing down and had to bump start it on the run, a very highly strung dramatic engine.

A few post down that thread,  there's reference to positive fuel pressure on pumped system such as Honda gold wing (lift pump from tank) that seems to infere that this would give PRESSURE at the jets ? When in reality the float chamber is always at atmospheric on gravity or pump fed carburation system.

It just seems odd there's so many points that don't add up.

[K2-K6]

With that description, a good place for a link to that 250/6 demo run https://youtu.be/CFbLXSoklI8 you can move the camera view round to look forward and include the rev counter too !

Stalled it more than i thought, note description in comments of "no idle jets" 

[JamesH]

Just following up on this, came across this old post on the US site:

http://forums.sohc4.net/index.php?topic=74546.0

Looking down the thread there's a post from "TwoTired" which pretty much backs up my understanding of how the pilot circuit works (TwoTired's words not mine):

"I have to disagree with Mark (Hondaman) on this point.  My understanding of physics refutes his explanation, as does my testing experience with an exhaust gas analyzer.

A given orifice size, flows a fluid based on the pressure differential across the orifice (assuming laminar flow of the fluid, which the standard jet operates within, in its normal regime).

The jets don't flow when the engine is not running because the pressure in the carb throats is the same as the pressure on the fuel in the carb bowls.  No pressure differential across the jet orifice equals no flow.
  When the piston falls on the intake stroke, less than atmospheric pressure is created, and this pressure loss (commonly referred to as vacuum pressure) then is presented to the carb throat.  The exit port for the pilot system exists within the carb throat.  Both the pilot jet and the air air jet feed this exit port.  The air jet presents atmospheric pressure and an air source, while the pilot jet presents fuel pushed by the atmospheric pressure on the fuel in the float bowls.

The air bleed screw regulates the inrush of air and also lowers the pressure differential applied to the pilot jet orifice.  With the air screw more open, both these factors reduce the fuel air ratio mixed in the emulsion tube (at the fuel jet exit) before it travels to the low pressure source in the carb throat, creating less fuel for the increased air volume, and reduced pressure differential applied to the fuel jet orifice.

The only way extra negative pressure in the pilot system could be created was if there was a venturi to increase the speed or flow of materials as there is in the carb throat, which it does not.  The flow of materials, air and fuel, in the pilot system is governed by physical principles and the pressure differential between what exists in the carb throat and the outside atmospheric pressure.  It really doesn't matter if the differentials are pulsed, the rates will be determined by the average over time.

I am sorry to disagree with Mark on this point.  But, my actual testing does not seem to support his theory (whether it is printed in his book or not).  My testing does exactly follow my understanding of the physics and science involved, and the hydrocarbons sniffed at the tailpipe show that the air bleed screw turned out makes the mixture leaner and turning it in makes the mixture richer.  (For the non-PD carbs found on our SOHC4s, of course.)

I hope I didn't put more typos in the above.

Cheers,"

[K2-K6]

The original description (that observers think is incorrect) looks remarkable in iits similarity to another type of carburettor.

The Webber DCOE behaves in this way, having a directly adjustable fuel passage to meter at idle as described. They are butterfly type throttle which when closed (open slightly for idle rpm) has a port that's already mixed for a/f ratio coming in on engine side of butterfly into that vacuum space. It has a taper screw directly in that entrance. It only adjusts FLOW at one point wuth the butterfly closed, with transient fuelling demand through additional holes (supplied by same circuit ) exposed along the carb bore as the butterfly arcs open to give transitional mixture in rising rpm, as each successive hole is passed by that butterfly edge when throttle is used so they are exposed to engine vacuum drop. On this system, screw out gives more fuel and visa-versa.

These SOHC carb have a fully transitioning idle circuit, balancing fuel air mass according to relationship of each bore component, air to fuel bore and their relationship in size and flow to the venturi volume.
It's effectively an air proportional valve that gives accumulating "slope" in idle and transition fuelling to bring the engine up into competent range of main jet running.

The two are fundamentally different.

[K2-K6]

Interesting view of intake here (and one for Ted  )  as it shows very clearly just how much mixture is ejected in "standoff" (the fuel mist outside intake with engine at full power) and usually contained within a good airbox system for it to be ingested by another cylinder https://youtu.be/hHjf9zhQIRk starts at about 1 min when he gets the throttle to full open.

[Johnwebley]

Oh the howling straight cut gears!


Remember them well

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