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Old 09-24-2006, 10:11 AM
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idle mixture screws

Hello.

I just installed a 770 Street Avenger on my 383. It fired right up but I noticed the idle was awful high. I backed out the idle speed screw all the way and it was still at 1800 RPM's. I then turned the idle mixture scews all the way in and eventually it died. I turned the idle mixture screws out a half turn and it idles right around 900 RPM's.

Where I am confused is that I read everywhere that the idle mixture screws should be adjusted to achieve highest vacuum. But if I turn the idle mixture screws out anymore the vacuum will go up but then the idle speed goes up to an unacceptable level.

I am confused by the contradiction. Am I ok where I have it now? I have a narrow band A/F ratio monitor and at idle is is just slightly rich and I am idling at 13 vacuum.

Thanks

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Old 09-24-2006, 10:28 AM
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Is there a fast idle screw on the choke mechanism? You should be able to kill the engine by closing the throttle even if the idle mixture is optimum.
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Old 09-24-2006, 10:39 AM
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idle

Adjust the idle mixture with a vacuum gauge , then adjust your idle speed to set your rpm,s. Former Jersey boy, Al
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Old 09-24-2006, 01:54 PM
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Yes,the primary butterflies should be way too open.Or even the secondary.Also check for a hidden air port leak,or gasket base leak.There is too much air going into the engine and you are trying to solve it on the idle mix needles.You will find it.Try the old trick.Bottle of soda,small hole to squirt water and with the engine running go around the carb slowly.The vacuum reading will change immediately when you find the leak.
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Old 09-24-2006, 03:35 PM
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Quote:
Originally Posted by onovakind67
Is there a fast idle screw on the choke mechanism? You should be able to kill the engine by closing the throttle even if the idle mixture is optimum.
It is possible to adjust the fast idle speed. But isn't the fast idle speed only when the choke is on? I haven't connected the choke to a 12 volt source yet.

Quote:
Originally Posted by chevmal
Adjust the idle mixture with a vacuum gauge , then adjust your idle speed to set your rpm,s. Former Jersey boy, Al
I tried to do that. But the idle speed is ALREADY set to the lowest speed and it is still idling extremely high when I adjust the mixture screws for highest vacuum.


Quote:
Originally Posted by paulo 1
Yes,the primary butterflies should be way too open.Or even the secondary.Also check for a hidden air port leak,or gasket base leak.There is too much air going into the engine and you are trying to solve it on the idle mix needles.You will find it.Try the old trick.Bottle of soda,small hole to squirt water and with the engine running go around the carb slowly.The vacuum reading will change immediately when you find the leak.
I checked for vacuum leaks and am confident there is none. HOw can I check of the butterflys are too wide open?

Thanks for all the help!
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Old 09-24-2006, 03:48 PM
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Ok.Butterfly or throttle.Remove the carby flip it upside down and have a close look.The secondaries should be almost completely closed,just rubbing.The primaries should be slightly open.Just enough to see light thru.Usually one turn from completely closed.You don't have a dragster type camshaft,do you?
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Old 09-24-2006, 04:49 PM
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Quote:
Originally Posted by paulo 1
Ok.Butterfly or throttle.Remove the carby flip it upside down and have a close look.The secondaries should be almost completely closed,just rubbing.The primaries should be slightly open.Just enough to see light thru.Usually one turn from completely closed.You don't have a dragster type camshaft,do you?
Ok gotcha. Will try to check them out tomorrow. Wish I would have noticed before I put it on. Anyway, a little more about the motor.

It's a 383 with vortec heads, 10.5:1 compression, RPM Air Gap intake and an XE274H cam. Nothing to radical.

Thanks for the help.
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Old 09-25-2006, 07:30 PM
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Quote:
Originally Posted by paulo 1
Ok.Butterfly or throttle.Remove the carby flip it upside down and have a close look.The secondaries should be almost completely closed,just rubbing.The primaries should be slightly open.Just enough to see light thru.Usually one turn from completely closed.You don't have a dragster type camshaft,do you?
Maybe this is the problem. I took the carb off tonight and can see light pretty much all the way around the secondary butterfly. But around the primary I cannot see light.

Here's a pic of the primary side. Does this side look ok? From what I've read the transfer slot is a little exposed. How do I adjust these?

Where do I go from here? Thanks again!
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Old 09-25-2006, 08:03 PM
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Ok Man.Apparently you do not have much experience.No problem.We will get it fixed.
1) Let's deal with it as if it was 2 carbs of 2 barrels each.If it is a double pumper (mechanical secondaries),disconnect and remove the rod that connects the primaries to the secondaries.Then look for a very small screw on the throttle plate,that contacts a small bracket on the secondary shaft,and limits how much it can fully close.It must be way too open.Unfasten it all the way until the butterflies close.

2) Now go back to the primaries.You will see a larger screw contacting a bracket to adjust idle speed.Unfasten it until the butterflies fully close and then turn it open about one full turn,as you see the shaft turn just a bit.

3) On the primary mixture plate you will find the screws to adjust air/fuel ratio.One per side.Fasten them until they stop and unfasten about 2 turns.

4) The slits are the transition circuit slots.They make the transition from idle to main circuit.As you adjust the idle circuit they automatically get set.

5)Run the engine with the vacuum gauge plugged and try to get the highest reading possible playing with the idle mixture screws.If the idle speed gets too high lower it on the larger screw by closing the butterflies to cut down air volume going into the engine.

If you get this done reconnect the secondaries and we will go from there.

Make sure fuel lines are secure and not leaking.
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Old 09-25-2006, 08:10 PM
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Thanks for taking all that time to write that Paulo! Your right too. I have had my car about 4 years and have made repairs and stuff that I never would have attempted before. But this is the first time messing with a carb. So I really appreciate the advice.

I will let you know how it goes.

Thanks again!
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Old 09-26-2006, 12:05 AM
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very good info.... I am very interested in hearing more on the secondaries, if you could?
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Old 09-26-2006, 07:22 AM
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72 Orange,

You have way too much of the primary transfer slot exposed at this point which why the RPM goes high when you add more fuel. You need to reset the butterflies, and re-install the carburetor. Normally on most street cars you'll want to set the rear butterflies so there is none of the transfer slot exposed, and set the primary butterflies so that you'll see about .020" of the slot below the butterfly (with the carburetor off and upside down). This will normally give you somewhere in the 800 to 900 RPM idle area. To adjust the rear butterflies (if they are open too far) there is an adjustment screw on the passenger side of the baseplate (usually coming up from the bottom side) that controls how far the rear butterflies are open at idle. Now your idle speed screw on the drivers side of the carburetor controls the amount of opening the primary butterflies are open at idle (provided that you're not on the fast idle cam). Turn your speed screw to set the primary butterflies so they expose about .020" (the transfer slot will look like a small box below the butterflies rather than a rectangle).

Also keep in mind if you do not have enough initial timing in the engine you may have to open the butterflies further than normal to operate. You'll probably want to be somewhere in the 14 to 16 degrees BTDC on your initial on this combination.
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Old 09-26-2006, 10:37 AM
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If the fast idle screw on the secondaries is not holding the butterflies open, wind the spring another turn that holds the secondaries closed.

I recently had to address a fast idling holley on my buddies car and that was all that was needed to fix the problem.
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Old 09-27-2006, 11:46 PM
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a few questions/ comments
1. how far open should the secondary throttle blades be on idle?
2. after tuning my idle/ air i have to turn the curb idle screw all the way out to get it to idle close to where i'd like it. is this right? i doubt it!
3. if i disconnect a vac port on the intake the vehicle will idle up. that doesnt seem right to me, i would take it that means i'm running rich.
4. if setting timing by vac. should i set idle first or timing first. this time i did idle then turned timing until max vac then adjust the curb idle and then turned down the timing to get it to idle at 1000 rpm... id like to see 800-900.
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Old 09-28-2006, 02:30 AM
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tnardo,

What are you tuning? Chevy, ford, mopar,... BB/SB...

Holley, Edelbrock, Q-junk...

comp ratio? Cam specs? ...

Your making a mistake that I once made. Adjusting your timing by vacuum. I learned of my mistake by folks on this site and haven't made it again.

With the vacuum disconnected from the distributor I set the initial timing to around 14-15 deg. before I tune the carb. Of course this will depend on what you are tuning. My experience is with stock to mild SB & BB Chevy's.

I have had acceptable results with making sure the secondaries close completely at idle. If you can't get your idle down by curb adjustment, I would suspect a vacuum leak, your secondaries are letting in too much air or too much timing.

I am in no way an expert on the subject but I do have some experience. If you want to learn a lot about carb tuning try doing an advanced search on posts by people like "tech @ bg" or some of the other names you will see frequently on carb and timing related threads. You'll probably find a weeks+ worth of reading.
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