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Exploding the Myths- Holley Idle/Mixture Screws

racerden

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Been asked this question alot:
"What happens when I screw in/out the metering block mixture screws; When I turn them in, is it leaning the idle fuel mixture or visa versa?"
Answer:
NO; idle mixture already happened inside the carb in the emulsion tubes. This mixture comes from:
Fuel- fuel well to the idle emulsion tube/passage orifice
Air- air bleeds top of main body. Their signal comes from the throttle body vertical slots at the throttle plates.

All you are doing when you turn the screws in or out is controlling the FLOW of the already mixed fuel & air. Basically they should be called idle flow screws, not mixture control screws. So if you have a carb that doesn't stall or severely stumble at idle when the screws are bottomed out (and all engine electrical & mechanical is where it should be) the mixture must be leaned internally.

Hope it helps!
Denny
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Infomaniac

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Hmmmmm............... :headscratch:

Good stuff but ... Don't think so man.

That would be correct if the engine was running ENTIRELY on what came out of the idle discharge holes. But it doesnt.

The fuel that is emulsified and coming out is mixed with the air that is going by the throttle blades. Screw the idle control screws in or out will allow more or less (emulsified) fuel to exit the carb. The amount of air is held constant by the throttle position so the AIR FUEL MIXTURE delivered to the engine is controlled by those screws.

The fuel is emulsified with the air bleeds so it flows through the internal passages easier. For example: suck any liquid through a straw. Then poke a tiny hole in the straw (much smaller than the opening of the straw) above the level of the liquid. It is much easier to suck the liquid out with the hole in the straw. Granted not as much flows through the straw but it is much easier.

This is necessary because the idle discharge nozzle is below the throttle plates and below the level of the fuel. The fuel has to be raised above the level of the fuel in the float chamber into the main well and then out the idle discharge. Otherwise it would just run out. ;)

If the engine doesnt shut down when you screw them in, they are not seating or the engine is getting fuel from somewhere else.

The main metering system has air bleeds to emulsify the fuel because the vacuum signal that makes that fuel flow is much weaker than the idle system. And the fuel flows much easier through the internal passages.
 
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racerden

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Hmmmmm............... :headscratch:

Good stuff but ... Don't think so man.

That would be correct if the engine was running ENTIRELY on what came out of the idle discharge holes. But it doesnt.

The fuel that is emulsified and coming out is mixed with the air that is going by the throttle blades. Screw the idle control screws in or out will allow more or less (emulsified) fuel to exit the carb. The amount of air is held constant by the throttle position so the AIR FUEL MIXTURE delivered to the engine is controlled by those screws.

The fuel is emulsified with the air bleeds so it flows through the internal passages easier. For example: suck any liquid through a straw. Then poke a tiny hole in the straw (much smaller than the opening of the straw) above the level of the liquid. It is much easier to suck the liquid out with the hole in the straw. Granted not as much flows through the straw but it is much easier.

This is necessary because the idle discharge nozzle is below the throttle plates and below the level of the fuel. The fuel has to be raised above the level of the fuel in the float chamber into the main well and then out the idle discharge. Otherwise it would just run out. ;)

If the engine doesnt shut down when you screw them in, they are not seating or the engine is getting fuel from somewhere else.

The main metering system has air bleeds to emulsify the fuel because the vacuum signal that makes that fuel flow is much weaker than the idle system. And the fuel flows much easier through the internal passages.
We're saying the same thing about air/fuel mixing..i/e emulsifying etc. On my post about the engine stumbling or shutting down I did throw in the qualifier saying IF everything else is in working order. This includes the carb integrity, i/e screw seating, main body warpage, floats etc.
Thanks for your thoughts.
Denny
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Infomaniac

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We're saying the same thing about air/fuel mixing..i/e emulsifying etc. On my post about the engine stumbling or shutting down I did throw in the qualifier saying IF everything else is in working order. This includes the carb integrity, i/e screw seating, main body warpage, floats etc.
Thanks for your thoughts.
Denny


OK Denny

I tried to splain it I guess.
;) ;)

Idle mixture is not because of the airbleeds. Idle mixture is a combination of the air going into the engine AND the emulsified fuel coming out of the carb. Controlled by the idle mixture screws.

And the screws DO control idle mixture, That is the only variable. The air going into the engine is constant at idle.
 

racerden

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I appreciate the everyones comments and direction. Others can and will benefit by what we have learned. For the record and one last time, this may clear things up about our discussions:

"Contrary to popular belief, the mixture needle does not control the idle circuit air/fuel ratio. Although this may simply be a matter of semantics, there is a difference between the mixture in the idle circuit itself and the mixture that ultimately reaches the intake manifold. The idle circuit calibration is determined by the diameters of the idle feed restriction and the idle air bleed. A 'feed restriction' is nothing moere than a metering jet for the idle system and the air bleed serves as an airflow regulating orifice. As the air/fuel emulsion exits the idle port it is mixed with small amounts of intake air that flows past the throttle plates. Varying the throttle plate opening (with the idle speed screw) increases or decreases both the amount of air that is allowed to flow into the intake manifold and the amount of pre-mixed air/fuel emulsion discharged at the idle port. Since these two idle mixture components are affected by degree of throttle opening, the ratio of intake air to idle mixture air/fuel is not drastically altered by changes in idle speed. Turning the idle mixture screw varies the volume of air/fuel emulsion discharged into the manifold, not the ratio of air to fuel in the emulsion. Therefore when a satisfactory idle cannot be achieved by tweaking the mixture screws richer (more idle air/fuel to intake air) or leaner (less), it can be assumed that the carburetor idle calibration is not within the range demanded by that particular engine.

The use of two four-barrel carburetors is an excellent example of such incompatability. With such an installation, the volume of air/fuel emulsion is effectively double that available from a single four-barrel mounted on the same engine. Even when the mixture screws are adjusted toward lean, there may simply be too much emulsified idle fuel flowing to allow a clean idle. In such cases the idle feed restrictions of both carburetors must be reduced, in cross section size,to lean the idle mixture to an acceptable level. These types of alterations are confined almost exclusively to highly specialized race applications. Satisfactory idle performance can, for the most part, be achieved by varying the settings of the idle mixture screws, and the idle speed screw."

And there 'ya have it.
Thanks again for everyone's input.
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racerden

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Any tips on getting my turbo motor not to hun?
HV..Do you mean 'hunt', like in idle hunting? If that's what you mean and it has what I call a "blower roll" the most common reason is it's just a bit fat at idle especially if it's a twin like mine. Occassionally I will pick this up when idling after a ski run but it smoothes out after a few seconds or so. I need to know if your'e running a trans and what turbo system you have. If this idle roll/hunt is what you are experiencing, Usually the simple fix is to lean out the mix screws a tad.
Hope it helps
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Infomaniac

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Yep and if that doesnt do it bigger idle air bleeds. lol sorry I couldnt resist. :p

I have seen too light of mechanical advance springs causing a surge or increasing the surge. Typically not the case but if the idle mixture adjustment doesnt fix it, it is somewhere else to look.


HV..Do you mean 'hunt', like in idle hunting? If that's what you mean and it has what I call a "blower roll" the most common reason is it's just a bit fat at idle especially if it's a twin like mine. Occassionally I will pick this up when idling after a ski run but it smoothes out after a few seconds or so. I need to know if your'e running a trans and what turbo system you have. If this idle roll/hunt is what you are experiencing, Usually the simple fix is to lean out the mix screws a tad.
Hope it helps
 

racerden

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Yep and if that doesnt do it bigger idle air bleeds. lol sorry I couldnt resist. :p

I have seen too light of mechanical advance springs causing a surge or increasing the surge. Typically not the case but if the idle mixture adjustment doesnt fix it, it is somewhere else to look.
Yep, and if that doesn't do it go straight to E85 (I couldn't resist either)...lol!..ya gotta love this gear head section eh?
PS
Hopefully if he's running some sort of distributor/mag set-up, it's locked out.
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HighVoltage329

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I have a Gentry setup in a 240 Hallett out drive (no trans). with the idle hunting it makes it hard to shift and hard on the drive.

Turbo240006.jpg
 
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racerden

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Most always, they hunt (lope) because they are a bit on the rich side at idle. Even mine will lope after a prolonged idle (1-2 minutes) in gear. If yours does it out-of-the-gate at all times, try screwing the mixture screws in a tad. Lastly, your Genrty set-up is a good one...they like idling around 1K RPM out of gear.
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