Mixture screw direction

[DomP]

Ive been in a discussion on the American site where a respected member insists that on an 069a carb turning the mixture screw in leans the carb?  This is contrary to everything ive been told

[Nurse Julie]

In for more fuel = richer, out for more air = leaner.
I've never understood the logic on this from the 'respected' member on the USA group either.....but he's incorrect.
Edit.....Im not referring to PD carbs, I've no idea about those.

[DomP]

He's applying Bernoilli's principle of flow dynamics apparently 🤷

[Nurse Julie]

He's applying Bernoilli's principle of flow dynamics apparently 🤷

He can apply any theory he likes but he's still incorrect 🙄🙄🙄🙄.

[K2-K6]

He's applying Bernoilli's principle of flow dynamics apparently 🤷

He's applying it backwards then 

Also dismissing the observable effect when it's turned in either direction as to what it does to mixture in practice 

[DomP]

Thank god for that then, I thought I'd been chasing my problem in the wrong direction!  I've finally got a big house project over the line so will have more time to look into this now...still plenty of summer left to get some miles in!

[McCabe-Thiele (Ted)]

He's applying Bernoilli's principle of flow dynamics apparently 🤷

Like many aspects of fluid dynamics it's a complex area, I've long forgotten my knowledge of fluid dynamics from my Chemical Engineering time at uni. I suspect that cherry picking aspects of Bernolli's principal that probably applies to laminar flow in a smooth pipe is far removed from a carburettor using a  small screw to introduce air into a vortex type fueling / mixture system. I suspect it depends on what side of the carburettor slider/butterfly the adjustment is being introduced as well as induction vacuum parameters.

Suffice to say I think the American is wrong in this instance, we here are doing it right as NJ states.

[scottly]

He's applying Bernoilli's principle of flow dynamics apparently 🤷

I suspect that cherry picking aspects of Bernolli's principal that probably applies to laminar flow in a smooth pipe is far removed from a carburettor using a  small screw to introduce air into a vortex type fueling / mixture system.

1+

[Robert W]

If the idle air mixture screw is on the air box side of the carb, adjusts the amount of air that mixes with the slow jet, which is a fixed orifice. Being a fixed orifice it will allow a certain amount of fuel through, depending on the current throttle vacuum condition. So it isn't 100% linear. The idle air screw then adjusts the amount of air that mixes wit the fuel. The air opening in the carb bell mouth is a fixed orifice also and will also not be 100% linear from the vacuum pull of the throttle open percentage. But together, they tend to work in tandem. this applies to the CB500 carbs such as the Keihin (type 1) 627B 71-73. So turn the air screw in means richer mixture.

From the info I've gathered over the years;
The CB550 74-76 also have Keihin type 1 carbs but a different model 022A.
The CB550 77 Keihin type PD 069A or X46A.
The CB550 78 Keihin type PD PD46C.

I'm not familiar with the 74-77 carbs, but I know that the CB550 78 has the idle mixture screws on the engine side of the carbs thus adjusts the amount of fuel. Keihin PD46C. These screws will adjust the fuel and not the air. So the pilot jet fuel flow gets trimmed down or up from it's fixed orifice jet while the air remains somewhat constant as an orifice depending again on the throttle opening vacuum. And they're set vertically upside down right next to the hot engine. (Pain) So turn the fuel screw in is leaner.

My thoughts, to the best of my knowledge which at times gets a bit scrambled especially after a few pints or whatnot.

[DomP]

Thanks everyone, I knew I was right but got shot down by a few people on there.

[DomP]

We were having a discussion on the American site but Hondaman has had a rant and then locked the thread.

[scottly]

I locked the thread after his rant.

[deltarider]

We were having a discussion on the American site but Hondaman has had a rant and then locked the thread.

I feel real sorry about all this, but this is what happens when people who know better, do not take their responsibility to correct errors regardless of the person in question and leave it to others. It has been going on for far too long and has lead to an unhealthy situation. Participating in a forum IMO brings a responsibility to 'sanitize' when needed.

[Rookster]

He's been posting the wrong info about the air screws for years now.  His response should tell you all you need to know. 

Scott

[Oddjob]

And yet, no one has mentioned his sources for his evidence of his opinions? Not read them myself as I’m on a phone but the sources appeared to be genuine Honda bulletins specifically for carbs of that era. Seen more than a few myself so I’m familiar with there type. So do they prove or disprove Mark?
I’m personally of the opinion that in is richer on these carbs BUT show me evidence I can’t dispute, as in from the manufacturer themselves, I’m willing to be convinced otherwise.

So what’s the verdict? Are people saying Honda were wrong or that Mark interpreted incorrectly or actually he’s right and we were all wrong.

Wasn’t much of a rant btw, if he hadn’t tagged it as a rant I’d just put that down as some sort of frustration coming to the surface. He mentioned no names, blamed no one. How is that worthy of having a thread locked?