Emissions Testing - Team Camaro Tech
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post #1 of 6 (permalink) Old Sep 17th, 04, 07:32 PM Thread Starter
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Paul
 
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I took the 69 through emissions to day. It passed, but the HC's were high (HC 403/CO .87) at idle, but fine under load (HC 126/CO .28). I am running a Holley TBI and a wide band O2 sensor. I watched the O2 sensor throughout the testing and the AFR was pretty much perfect the entire time (between 14.6 and 15.0).

I guess I'm wondering what would cause my HC's to be so high during the idle test, but still show near perfect AFR? Any ideas?

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post #2 of 6 (permalink) Old Sep 17th, 04, 07:48 PM
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What is your timing at idle?

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post #3 of 6 (permalink) Old Sep 18th, 04, 04:01 AM
a67
 
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The AFR of between 14.6 and 15.0 is a little lean for idle. For lower HC reduce the timing a few degrees and richen it up a tad. This will cause a better burn and lower the HC content.

Bob.
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post #4 of 6 (permalink) Old Sep 18th, 04, 06:54 AM Thread Starter
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My timing is set at 34 degrees total with a large body vacuum advance GM HEI....You know, the last time I set the timing I only set the total timing, I didn't even look at initial. I am assuming though that puts it at about 14 degrees plus the vacuum advance of what....about 8 for a total of around 22 at idle???

I was reading some exhaust gas analysis stuff on the web and I read that excessive carbon buildup can cause high HC readings...I wonder if that could be it?

I'm really interested in what you guys think would cause high HC's at idle when the AFR is about right. 400+ HCs at idle is really high, compared to the 126 under load.

So, Bob you are saying that a slightly lean idle can cause the HC's to be really high? Sheesh, I could see the HC's being a little off if the AFR was slightly lean, but 400+ seems high considering that the AFR was pretty close.

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post #5 of 6 (permalink) Old Sep 18th, 04, 08:00 AM
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Ill assume you have no cat or other smog era devices. A lean idle can cause a slight lean missfire which will cause HC to rise while CO will remain low, this is assuming that the car is in good running order and has no other problems. A common cure is to either richen up the air/fuel mixture to get rid of the lean missfire or retard the timing so the combustion chamber gets hotter which allows the fuel to burn more completely.

For best idle quality I would say richen the mixture up a little. 13.5 to 1 is where most engines run the best.

70 Camaro, 355, th350, 4.10 posi.
74 Trans Am, 400, th400, 2.73 posi.
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post #6 of 6 (permalink) Old Sep 18th, 04, 10:47 AM Thread Starter
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Paul
 
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Yes, this is a 69 camaro with dual exhaust and no smog stuff.

There is no way that I believe that I would be having a lean misfire while my AFR is 14.6 - 15.0. I have encountered lean misfires, but at AFR's of around 17:1 and leaner.


My engine idles just fine. The question that I have is why would I have high HC's (unburned fuel) when my AFR is near perfect? Unless you can convince me somehow that it is possible to encounter a lean misfire at AFR's of 14.6 - 15.0, there must be another reason.

One possible cause that I was thinking about is would it be possible that I have a fouled plug or something on the opposite cylinder bank? My O2 sensor is mounted in the drivers header, maybe there is an issue on the passenger side?

How about timing? How does that affect things? If the AFR is near perfect doesn't that mean that there should be very low HC's regardless of the timing?

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