Topic: Setting Air Screws on cb750.

[BenelliSEI]

FYI…… Setting air screw comments from another post. Three or four members (including me) suggested they just set them to spec….. Hondaman’s the last post.
 
Quote from: BenelliSEI on March 26, 2024, 10:01:02 AM
Quote from: Galactica on March 26, 2024, 08:11:21 AM
Carb question.

I’ve always had difficulty setting air/fuel mixtures, whether using an air screw or a fuel screw.

Using the tachometer on the bike or a vacuum gauge, I not really able to discern changes in rpm when adjusting the screws.

Can anyone point me to a good tutorial on this?  Is there a good post on this site anywhere?

Cheers,
Ross

Ross…. You’re not the only one! I’ve decided I must be an idiot. I never see much difference either. I set them to the recommendation and leave them alone!


+1! It isn't possible to listen to the engine and adjust the air screws for 'best idle', for example, primarily because the cylinders interact. It's not like a single, nor even a twin. Also, 50+ years later the carbs themselves have often receded where the air screws seat, so they must be opened less than the 1970s spec to get the same mixture. I've long used 'slightly less than 1 turn' for the air screw settings.

[Scootch]

I'm glad to hear this because I've never been able to hear a difference. I set them for about 7/8 and call it good.

[PeWe]

I have noticed differences when cruising around in ca 70-100kmh.

I did some air screw rides with my my K6 carbs 086A.

I set the screws way wrong to feel the difference. Opened 1 1/2 turn.
Result: Bike acted as early indication to change petcock to reserve.

Then reduced by 1/8 each time. OK at 1 turn out.
Small hesitation at take offs reduced air screws another 1/8.

Same bike has now 064A carbs with richer needles as default. Taper is richer so 1 step lower.

I have not played with air screws that much yet. Only as described.
(I'll test slightly smaller main jets next time, from 115 to 112.)

Need a ride on old jetting first. Change MJ after a 200km ride. Another 200km ride asap to feel the difference and check plugs again.
Photos when plugs are cold to see the difference. No pitch black nor white grey thanks.

Change air screws must be done on rides.

Bring a screwdriver with you. Change after ca 50km and feel the difference.

After a first change it can be enough to change each 10-20km.

Alternate slow take offs with slow riding shifting up all gears and cruise. Not much more than 100kmh.

My Mikuni TMR32 with fuel screws reacts very much.
Steps of 1/8 is much.  Open screw give more fuel.

Hesitations during light throttle increase (lean)or rich d-d-d sound at slow roll-offs.
Too rich can d-d-d at slow roll-on too.

Even here important to adjust during a ride. I have adjusted a lot!!
Make sure that engine is fully warmed up.

A mistake to adjust when cold to get too rich after 50km further.

[MauiK3]

I've always felt that I'm the only one! I could never actually detect much difference except at extreme settings.

[BenelliSEI]

I’m going to try what Per does and see if I can detect a difference. Up to now I’ve been clueless and just set at the suggested number of turns, for whatever I’m working on!

[M 750K6]

Are you turning the screw in or out to richen?

[HondaMan]

Are you turning the screw in or out to richen?

Out = richer

[M 750K6]

I've seen your explanation for this, which makes good sense, but I am finding screwing in the screw is giving me a richer tickover / low throttle and sooting the plugs.

Damndest thing!

[newday777]

I've seen your explanation for this, which makes good sense, but I am finding screwing in the screw is giving me a richer tickover / low throttle and sooting the plugs.

Damndest thing!

On a 750 with round top carbs or your 500 four?

[M 750K6]

750K6

Seems to behave like a straightforward air screw. I checked carb synch, timing, points and valve settings, to see if I had another problem. They seem spot on.

As Mark I think, said previously, they have a tendency to run a bit rich at tickover, but less so as I unscrew it a touch further and that's when it stops bogging on opening up the throttle. In essence, despite best advice, I'm using it like an airscrew.

[HondaMan]

I've seen your explanation for this, which makes good sense, but I am finding screwing in the screw is giving me a richer tickover / low throttle and sooting the plugs.

Damndest thing!

What actually happens is: as the screws go far enough inward to mix below 15:1 the cylinder starts missing, not burning the fuel on hand. It collects quickly (3-4 engine intake cycles) and then fires, but now it has insufficient air to burn several intake stroke's worth of fuel. So, the plugs (and exhaust) become rich once past the stoichiometric point, and wet, and then they quickly get black.

Somewhere near 1/2 turn out (slightly more than that) is where the air simply gets pinched off enough that it will not atomize the fuel via those tiny holes in the idle jet. This is when the mix "goes rich", but in truth it is being ingested soaking wet, unaerated, and unable to burn. So, the internet has decided this means IN=RICH, despite physics.

The internet is like that....