Quick fuel air mixture screw setting

Old July 21st, 2013, 03:02 PM
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Quick fuel air mixture screw setting

I have a Quick fuels "slayer" 750 CFM.
And I only have to turn the air mixture screws out 1/2 a turn and it idles up real nice and gives me my highest vacuum reading.
Im used to having them out about 1 1/2 on a regular Holley.
I guess this is OK as long as its running good and not real rich out the tail pipe.
They also say to set the floats to the middle of the sight glass but a Holley says to the bottom???
Im not having problems just seeing if it sounds as good to you all as it looks good to me????
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Old July 21st, 2013, 05:11 PM
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If it works at 1/2 a turn then so be it. Smoothest idle and highest vacuum is what you want so it sounds like it's good.

I always set the float to the low point in the sight plug. Back in the day you had no sight plug so you set it until it just dribbled out the hole. My Demons actually have a line cast into the fuel bowl that shows the correct level. It's about a 1/4 - 1/3 of the way up the glass. That setting is not as critical unless you're having problems with fuel coming out the vent under acceleration. I like to keep them low but have plenty of pressure and volume to keep it filled.
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Old July 22nd, 2013, 05:24 AM
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Originally Posted by TripDeuces View Post
If it works at 1/2 a turn then so be it. Smoothest idle and highest vacuum is what you want so it sounds like it's good.
Sorry but that's not necessarily correct. Yes the idle may be fine but I'll bet your off idle cruise is fat.
Make sure the power valve is half the vacuum reading you have at idle in gear. In other words, if you have 12" of vacuum in gear then you should have a 6.0 Power valve etc.
Also that carb has replaceable idle air bleeds. You may want to go up a size or two. That should bring your idle mixture screw setting to 1-1 1/2 turns out. That's normal. Basically it would seem now that your low speed circuit is on the fat side.
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Old July 22nd, 2013, 06:01 AM
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His low speed air bleeds should not even be used at idle. That happens through the transition circuit. Then he'd have a different problem. If the transition slot is uncovered it could cause this.

SI442, how big is the cam?
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Old July 22nd, 2013, 06:22 AM
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While I'm on the topic there are idle air bleeds. They need to be clean and clear.

This conversation from another forum may help too.

"for proper idle mixture control... with your foot off the throttle and the choke all the way open where fast idle cam screw is no longer holding the throttle linkage open...

the end of the idle transition slots should just barely be exposed.. so all the fuel at idle should be coming through the round openings and past the end of the idle mixture screw... there is an idle air bleed at the top.. that allows the idle circuit to pull air in also.. prevents an overly rich idle mixture...

when you tip into the throttle slightly... the end of the throttle blades exposes more of the idle transition slots... this starts fuel pulling out the idle transition slots to feed the engine as there is not enough air flow through the booster venturi to lower pressure enough to lift fuel up and over where the main circuits take over.

at part throttle the idle transition circuits still feed fuel into the engine.. so they are very important to have working...

on holleys under one of the plugs in the top of the primary metering block is the idle transition feed restriction... but the idle air bleeds are involved in this also...

again to prevent the idle transition circuit from pulling straight fuel.. the idle air bleed.. allows some air into the idle circuit.. this gives the circuit proportionality... more exposure of the idle transition slot.. more pull on the circuit and more fuel is pulled over as the pressure below the idle air bleed is lowered... this pulls more fuel for the idle transition.. .


the high speed circuits work in the same way... the high speed air bleed sizing changes the amount of fuel in relation to the speed of the air through the booster.. faster air movement.. lower pressure.. more fuel is pulled up and over... "
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Old July 22nd, 2013, 06:28 AM
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Originally Posted by TripDeuces View Post
His low speed air bleeds should not even be used at idle. That happens through the transition circuit. Then he'd have a different problem. If the transition slot is uncovered it could cause this.

SI442, how big is the cam?
I think maybe you're confused with low speed bleeds.
Per a QF tech guy;

If best idle is achieved with only a 1/2 turn then the idle air bleeds need to be replaced with ones of a larger size. They mix the air and fuel prior to it going thru the rest of the idle circuit.

The idle air bleeds are direct correlation to the idle circuit and are located in the main body of the carb. They effect the idle much more than anything else. From there the QF has replaceable restrictors in the metering block that will effect the part throttle air/fuel. 2 separate circuits in the QF stuff, that's one reason why I like them over a Holley.

Last edited by cutlassefi; July 22nd, 2013 at 06:35 AM.
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Old July 22nd, 2013, 06:31 AM
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The idle air bleeds could also be clogged
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Old July 22nd, 2013, 09:17 AM
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The cam was in the engine when I traded for the car, it lopes but will hold in gear fine and has 16 of vacuum at idle in park.
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