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76 390 wont pass emmisions

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Old 09-01-2009 | 01:38 PM
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76 390 wont pass emmisions

Didnt pass on the first test so I changed plugs,wires, cap and rotor and added a cleaner to the gas. Put about 50 miles on it and just went back and retested and still didnt pass. In fact it was worse this time. I'm thinking I need to lean out the carb. Any suggestions or other ideas?
 
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Old 09-01-2009 | 02:09 PM
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mark a.
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What state do you live in ? I'd think there should be some type of exemptions for a truck that old.
 
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Old 09-01-2009 | 02:16 PM
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Profile says Utah.

Did they give you a printout of your results vs. the specs?

High HC = rich condition for example. How far off you are would help in figuring what adjustments to make.

High CO = dead cat--but wait, if you also had high HC and missed CO by just a little, the cat may still be good, and the fix is in the HC which is leaning it out a bit at the revs affected. (if you have a cat...)

More info = better help.
 
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Old 09-01-2009 | 02:19 PM
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Can you post the specific readings eg....... Nox, co2? That will better help with what's wrong.
 
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Old 09-01-2009 | 02:24 PM
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Utah - could be an altitude problem - possibly running too rich... I'm thinking you're on the right track by leaning out the carb... Sometimes the strangest damn things can affect emissions testing though - I once fought this issue in King County WA with the wife's '76 Dodge van - turned out the problem was the coil was going bad (it was showing signs of leakage from around the output terminal)...
 
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Old 09-01-2009 | 07:03 PM
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First test passed HC at idle and high speed test but failed CO by 1.92% high speed and 3.76% idle. Retest failed HC and was 2.53% over on CO high speed and 5.1% over at idle. I'm thinking I should run this half full tank of gas out with the additive. I was thinking a coil problem also. Sometimes just doesnt want to fire even though it truns over OK.
 
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Old 09-01-2009 | 07:13 PM
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There's two things going on.

High HC's points right at a rich condition.

High CO can not only be the catalytic converter not effectively dealing with the CO, but also CO is a by-product of an inefficient burn. It just might be too much CO for the cat to deal with.

Inefficient burn is caused by all sorts of things, but if you are running rich, leaning it up will not only bring down the HC's but also the CO. To a point. Too lean, and the CO will start going up again.
 
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Old 09-01-2009 | 08:45 PM
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I have plenty of experience with this until my truck qualified for Classic plates.

Basically everything you will end up doing to past the test will make the engine run like butt, but here it is:

You'll need to lean out the mixture screws as much as possible and you'll need to set the timing back to stock i.e. 6 degrees. Run the truck hard, like on a highway, pull over and squirt some water down the throat of the carb, keep the rpms up so the engine will not die. You'll end up dumping like a quart or 2 into the engine, now run it hard again and go straight to the emissions test.

You should pass. Once the test is over you can either go home if it isn;t too far or go around the corner and reset you mixture screws and reset your timing.

Josh
 
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Old 09-01-2009 | 09:42 PM
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Originally Posted by mark a.
What state do you live in ? I'd think there should be some type of exemptions for a truck that old.
I agree! I VA it would be an antique!
 
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Old 09-01-2009 | 09:46 PM
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In Colorado you still need to pass emissions in order to purchase the Classic Plates once the truck is 25 years old. After that, as long as you keep the tags in good standing you'll never have to mess with emissions again.

Of course, being the dummy I am, I struggled with emissions from 1993 to 2000, got Classic plates and then moved to rural Weld County which doesn't require emissions testing anyway... D'Oh!

Josh
 
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Old 09-01-2009 | 10:02 PM
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Im from Utah also. It sounds like your timing is off just a little. Timing controls your HC's and Carb adjustment controls your CO's. Set timing to 8 degrees, turn your mixture screws all the way in then back them out 2 turns for the motorcraft 2 barrel, and 11/2 turns for edelbrock 600 4 barrel. If you have a motorcraft 4 barrel throw it in the garbage and get your hands on an edelbrock you will love it. Hope this helps.
 
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Old 09-01-2009 | 11:01 PM
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In Texas, 1985 or older, no emissions testing. Did they even have cat converters on trucks in 76? 74 Bronco didn't. 78 Bronco didn't. 81 Bronco did.
 
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Old 09-02-2009 | 01:09 AM
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Dumping water in the engine when revving is a good way to get rid of carbon but i dont think that would be an issue. As far as carb adjustment goes, turn one side in all the way, the truck should want to die. turn it out until it smoothes out, go 1/4 turn past that. Repeat with other side. Also, make sure its idleing at the right rpm, 740 iirc.
 




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