Mixture screws...which one is fuel?

[Dave_in_Texas]

On the 2100 Autolite 2 BBL carb, which mixuture screw is the fuel and which is the air?

Thanks,

Dave

 

[oleguy74]

both!!!carefully close both turn out 1 1/2 turns to begin with.set idle low as possible to keep running.turn one in till nearly stalls,then open till runs smooth.do same with other one.

 

[AKwheeler]

The two screws that are on the front of the carb just above the flange are you idle mixture screws. If you are trying to adjust the air/fuel mixture at warm idle these are the screws you want to adjust. Initial adjustment is turn in until lightly seated, turn out 1.5 turns. This adjustment is made after the engine is fully warmed up, and the choke is completely open. I believe factory adjustment is for 13.2-13.8 A/F, but if you don't have an air/fuel gauge or gas analyzer do what everyone else does (I have one ;D ). Adjust for best idle. I find that doing that then leaning it out a bit brings it pretty close.

As far as fuel screw and air screw: You adjust the mixture screws, which is a blend of fuel and air (added at the air bleeds), so the same screw sometimes is called the air bleed screw, air mixture screw, fuel mixture screw and such. You can adjust the fast idle, base idle, choke and tweak the accelerator pump. The rest is "magic".

 

[Dave_in_Texas]

Cool...thanks for the reply! I always thought one was the air and one was the fuel.

Dave

 

[kbldawg]

I rebuilt my carb today and noticed that one of my mixture screws is longer than the other. Is that common?

Also, this could be related to the question above, but...

I tried running mine out 1.5 times each and I had major flooding. I turned them in all the way, then back out just a half a turn and it cranked right up. So, even thought 1 and half turns is standard, be prepared to try just about anything to get it right.

 

[Dave_in_Texas]

I think when I rebuilt my carb the screws were the same length. Anyone else able to chime in?

Dave

 

[broncnaz]

I rebuilt my carb today and noticed that one of my mixture screws is longer than the other. Is that common?

Also, this could be related to the question above, but...

I tried running mine out 1.5 times each and I had major flooding. I turned them in all the way, then back out just a half a turn and it cranked right up. So, even thought 1 and half turns is standard, be prepared to try just about anything to get it right.

Its not common. I'd say someone may have lost a or both original screws and replaced one or both so now you have one that is different sized. Its really no big deal as long as the taper on the needle part is similar to the original one.
1 1/2 turns out is only a base line although usually its very close to the final setting for most people. These screws have zero to do with flooding if your carb is flooding your problem is elsewhere in the carb not the mixture screws.

 

[kbldawg]

Well, I believe you, but...obviously it had something to do with it. Perhaps not directly, but indirectly.

It wasn't until I started adjusting those screws that it started flooding. After I turned them in all the way and back out half a turn, it stopped flooding.

Seems aweful coincidental to me.

 

[broncnaz]

Maybe its your definition of flooding. Typically flooding means that fuel is running out the bowl vents and into the carb throat. Basically flooding the engine with fuel making it die and very hard to restart until the fuel evaporates.
In most carbs Idle mixture screws only adjust the amount of bleed air allowed through the idle passage. Theres really no way for the idle screws to affect fuel level.

 

[old number 8]

The deal with the screws having to be turned in so far is usually due to someone over-tightening them. This enlarges the metered fuel passage into the airhorn, making it flow more fuel than intended. It also makes adjusting them very touchy.

 

[kbldawg]

Maybe its your definition of flooding. Typically flooding means that fuel is running out the bowl vents and into the carb throat. Basically flooding the engine with fuel making it die and very hard to restart until the fuel evaporates.
In most carbs Idle mixture screws only adjust the amount of bleed air allowed through the idle passage. Theres really no way for the idle screws to affect fuel level.

That could be. I admit I have just a slight better understanding of carbs than a rock.

I'm just happy to know how to stop it.