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Idle mixture screw adjustment

October 15 2012 at 7:11 AM
Bob  (Login F150Bob)
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I rebuilt the Holley 4160 on my stock 1976 390 last fall. I tuned the carb following the instructions, but just ran it with the mixture screws 1.5 turns out becuase I didn't have a vacuum gage at the time. The truck has been running fine, but I bought a vacuum gage and decided to see if I could get the carb tuned better. I hooked up the gage to a vacuum port at the base of the carb (manifold vacuum port) and I get around 18 inches hg. Then I started fiddling with the mixtures screws trying to find 'peak' vacuum and I discover I can turn the screws all the way in and the vacuum increases to about 20 inches and the truck smoothes out a bit and does not stall. I backed them out half a turn and it runs about the same as all the way in, but better/smoother than 1.5 turns out, so I left them there. I guess I have a vacuum leak. I checked all the vacuum lines coming off the carb for leaks, pulling all the lines and installing plugs and there was no change. The lines coming off the tee at the back of the intake manifold also seem to be fine. Any ideas where my leak might be? Could there be something messed up inside the carb that is causing this problem?

 
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BattlestarGalactic
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Could be

October 15 2012, 7:23 AM 

your secondaries are open too far. It does actually draw some air/fuel even at idle. If it draws too much(blades opened too far), then your primary circuit mixture will not function properly.

There is a small set screw on the bottom side of carb, that sets blade opening. Turn it over, unscrew it 1/4 turn and try again. Just a "try it and see" thing. You'll find a sweet spot where your primary mixture screw will work properly(should stall if turned all the way in).

Could be a vacuum leak also? Depends on all what you disturbed.


LarryK

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1964 Galaxie 2dr 390/6-71/4spd
1964 Country Sedan Wagon 428/5spd
1969 F100 428CJ/4spd
1989 F100 302/AOD
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Faron Rhoads
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Well your Backwards in your thinking , UNLESS

October 15 2012, 7:23 AM 

Its one of the Smog Holley's that just changed how much AIR the Idle circuit sees , Normal Holley's adjust the TOTAL mixture with air AND fuel , so turning it all the way in Leans the mixture ( Opposite of what a Vacuum leak would show ) , so in other words its a tad RICH , NOT Lean , still I bet its just on the verge , Id drop Main Jets down 3 numbers and readjust , also check for a leaky Power valve while you are in the carb , happy tuning

 
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Leland
(Login lhstanford)
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I just had this experience with my Holley

October 15 2012, 7:37 AM 

I found out it's the smog Holley which has the reversed air mixture adj. I dont like it cause it's not very responsive to adjustment and putting my old 4100 carb back on.

 
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Bob
(Login F150Bob)
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Smog Holley

October 15 2012, 8:02 AM 

How do I determine if it is a "smog Holley?" I believe this is the stock 1976 4v that came with the truck when new and it was a California truck. I will look up the list number when I get home if that helps.

 
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Falcon67
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Idle

October 15 2012, 7:35 AM 

Check the primary throttle plates - usually what you are seeing is the primary plates open too far. Should be maybe .020~.030 of the transfer slot showing. If you have the idle set too high, or the idle screw cranked in too far to set your idle that will do it. Do not change the jets - jets have almost nothing to do with the idle mix. This is also assuming a standard Holley, not one of the "econo" type engineered in the 70s. As noted above, the idle setting on those is bass-ackwards.

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Owner built, owner abused.

 
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Dave Eames
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Re Idle. Recheck the float level, check every vacuum line under

October 15 2012, 8:11 AM 

the hood for cracks/splits, etc and replace as needed. Find out if the jets are stock or have been changed out and if so return to stock. Recheck the float level and get a can of spray carb cleaner and while holding the throttle up to about 2k rpm's spray/douche the air bleeds in the front and back of the main body; they'll goober up over time and affect the idle. Gently seat the idle mixture screws while counting the number of turns in to seated, remove them and write down the number of turns. Flush the circuit with the carb cleaner and follow up with a blast of compressed/canned air. Reinstall the mixture screws, adjust and see if this makes a difference. The secondary set screw isn't gonna turn itself and hold the sec's open nor is the curb idle screw going to do the same since last fall. Don't change the jets in order to mask/cure? an internal issue. If nothing changes you may have to go back in to correct gasket shrinkage, douche, etc. as fuel will corrupt gaskets over time.
Eames.

 
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Faron Rhoads
(Login Faron)
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He didnt say it was a change in parts , Just the way he tuned it

October 15 2012, 8:49 AM 

He just set it to a standard 1.5 and left it there , now he is using a Vacuum gauge , I concede Main jets do not effect Idle mixture by much , BUT he hadn't posted What Jets he had a List Number and having the Stock jets , ( and what is in it know will obviously help ), all the above are good points to consider , without knowing all the variables , and specs on his particular combo.

 
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Bob
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Jets and more

October 15 2012, 9:40 AM 

I don't know the jet size, but I am certain they have not been replaced and are stock for this engine. The truck has been in my family since new in 1976. It is a California truck and had full emissions on it, so I suspect it is a smog Holley which may explain why it doesn't behave like I expect it to (although I really don't have much experience tuning carbs, so my expectations dont mean much, LOL).

 
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Dave Eames
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Might be either a 6946-1 or 6947 Holley 4160

October 15 2012, 3:49 PM 

as both of these numbers were installed on '76 390's in Cal. If it was/is a reverse idle adjusted carb there shoulda been an "arrow" decal on the metering block over the mixture screws pointing ccw with word "lean" printed on it.
Eames.

 
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Roberson
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Try this also

October 15 2012, 4:14 PM 

If the truck hasn't been driven in a while--unhook all your vac hoses and plug the spots where the hoses pull vacuum--including the brake booster. retune tha carb and see what you come up with on your vac gauge. then if it's better, start replacing vac hoses one by one and see after each hose is hooked back up if the readings change--had a brake booster drive me nuts like this once--just a little vac leak coming out of the booster and couln't spot it until I did this.

 
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