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Old 10-11-2013, 07:11 PM
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Adjusting idle mixture screws

Hi,
I'm trying to adjust the mixture screws on my Edelbrock 1406. It's sitting on top of a performer intake on my cammed sbc 305. The problem is the exhaust smells rich. I ran both the mixture screws all the way in. I thought this would kill the engine but it kept running. I backed both of them out about 2 turns and went for a test drive. Now sometimes when I let off the gas it backfires out the exhaust. It's really slow off the line also. I'm not sure what to do now

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Old 10-11-2013, 07:23 PM
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Get a vacuum guage and adjust the idle mixture screws to the highest vacuum.
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Old 10-11-2013, 07:39 PM
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When I tried that the gauge showed 17hg no matter where I set the screws. I hooked up the the big vacuum port on the front of the carb.
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Old 10-11-2013, 07:46 PM
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Put a fuel regulator on it and set the fuel pressure at 5.5 psi.
THen adjust your mixture screws.
If it still does the same thing crank your timing up to 16 degrees initial, slow down the idle and then adjust them.
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Old 10-11-2013, 07:49 PM
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If you screw them all the way in at idle it should cut off the air supply and the engine will die. If it does not kill the engine then its getting air from somewhere else. The most likely cause is a vacuum leak (which needs to be fixed), but there are other possibilities.

If you have an aggressive cam and have set the idle up high to keep it running you may be pulling air off the main circuit. Set the idle down low and then use the mixture screws to get the best idle. Also verify that your base timing is set to something reasonable (about 8-12 BTDC should work), and disconnect the vacuum advance line at the distributor and plug it tightly.

Bruce
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Old 10-11-2013, 07:59 PM
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I'll try a pressure regulator. Timing is set at 18 degrees and rpm is 1000 at idle and drops to 750 rpm in drive
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Old 10-11-2013, 08:04 PM
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Since you did mention it running rich, it may be worth verifying that your choke is working properly. Also make sure that your throttle is not stuck open at all and that the return spring is functioning properly. Good luck.
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Old 10-11-2013, 08:08 PM
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Choke is functionally properly.
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Old 10-11-2013, 09:30 PM
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What's the cam specs?


Duke
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Old 10-11-2013, 09:40 PM
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I lost the cam specs when I moved and can't remember which cam it was. It's a speedpro cam.
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Old 10-11-2013, 10:14 PM
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Quote:
Originally Posted by Tri5truck View Post
Hi,
I'm trying to adjust the mixture screws on my Edelbrock 1406. It's sitting on top of a performer intake on my cammed sbc 305. The problem is the exhaust smells rich. I ran both the mixture screws all the way in. I thought this would kill the engine but it kept running. I backed both of them out about 2 turns and went for a test drive. Now sometimes when I let off the gas it backfires out the exhaust. It's really slow off the line also. I'm not sure what to do now
It really does sound like a vacuum leak. Check that first.
So assuming that checks out
Hook your vacuum advance up to a manifold source, IIRC its the drivers side port on the front of the Carb
This will increase your idle speed, now turn it back down with the idle screw to your 8-900rpm idle speed
This should now give you control of your mixture screws. Start at 1.5 turns each and use your vacuum guage, small changes, keep them even.
Good luck

Dave
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Old 10-11-2013, 11:04 PM
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Quote:
Originally Posted by RWENUTS View Post
Put a fuel regulator on it and set the fuel pressure at 5.5 psi.
You may have to drop the pressure to 4 or 4 1/2 lbs. Whatever works. More pressure will not make more horsepower. You need only enough pressure to fill the bowl and keep it full. 5 lbs or more can easily unseat the needle and blow raw fuel into the intake manifold, creating a tuning nightmare.
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Old 10-12-2013, 05:00 AM
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have you set your idle transfer slot correctly? if too much of the transfer slot is exposed it can reduce the effect of the mixture screws.
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Old 10-12-2013, 12:29 PM
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I've never adjust idle transfer slots. How do I check those?
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Old 10-12-2013, 12:43 PM
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Take the carb off, flip it over and set it like in this link pic.
Demon Carburetion 625 Carburetor - Street Demon - Mustang Monthly Magazine
Or set your mixture screws at 1 1/2 turns out, turn down your idle to bare minimum, adjust your mixture screws in slowly until the idle stumbles, back off the screws 1/4 turn, shut her down and check how many turns from bottom your mixture screws are set at. Anywhere from 1 to 2 turns will put you in a well set transfer slot.
However if you can turn either screw in all the way with no idle change then you need to speed up the idle without using the idle screw on the carb. Hook vac advance to manifold or increase your timing. THen try again.
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