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Emissions test - HC too high

 
omgang
V-6 Camaros rule!
Posts: 54
Joined: 12/04
Posted: 04/12/06 10:39 AM

It's time for me to get my '76 Blazer through OR emissions test.  The engine is the new (~1000 miles) 406 impersonator I built over the winter.  My first time through, the HC (~600) and CO (~3.2) were both too high.  I leaned out the idle mixture by approx. 2/3 of a turn on each screw (Edelbrock 750) and raised my idle speed slightly to ~925 rpm (neutral).  I went through again today and the HC are now even higher (~1000) which I did not expect.  The CO is now within limits (~1.8).


Should I be looking for an idle misfire?


The carb is new and the only change from stock settings is I swapped needles to increase cruise AND power steps by 4% (per Edelbrock manual).  The plugs still look relatively lean (mostly white with just a slight hint of tan).


OM Gang

 

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omgang
V-6 Camaros rule!
Posts: 54
Joined: 12/04
Posted: 04/13/06 08:59 AM

I forgot to mention that my initial timing is at 15 deg.  The new Mallory dist. I installed has 21 degrees of advance in it, fixed (yeah, tell me about it - I should have spent the extra money).  I'm wondering if backing off initial timing a few degrees might help.


OM Gang

 

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omgang
V-6 Camaros rule!
Posts: 54
Joined: 12/04
Posted: 04/14/06 01:39 PM

Correction - my distributor only has 20 degrees of advance in it so I was running 16 initial (although when I checked it today it was actually at 17.5).  I backed initial timing down to 12 degrees and went back for another test.  The CO counts dropped even further (now very lean) but the HC counts only dropped a bit.  the truck actually runs significantly better with the initial timing set where it is now (starts better, idles better, runs smoother).  The problem is now I've only got 32 degrees total - guess it's time to buck up for a good distributor.


I guess I'll try backing initial down to 8 degrees and see if that'll get the HC counts down low enough.  Would a different heat range plug have any effect?  I'm currently running autolite 25's.  Anyone have any other suggestions?


OM Gang

 

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danoman3
V-6 Camaros rule!
Posts: 28
Joined: 12/05
Posted: 04/27/06 01:30 PM

Hello,


First thing to ask is where do you live, high alt, or low. If I read your post correctly the carb is an edelbrock, I have one and it was a bit rich out of the box but I live at 6500 feet so almost anything I get is rich. My carb came with a chart and I used it as a guide to lean mine out a bit, still not quite there yet as I dont have an o2 sensor kit , I just have to drive it around and FEEL it out. As I sure you already know high HC is unburned fuel so look at what causes that and go from there. Since I dont' know what your limit is for HC I don't know how bad bad is, though sounds like youre close.  If the CO goes too low it can cause a lean idle miss that can be hard to detect which will raise HC. I'd leave the timing where it ran the best and gap the plugs a little wider and run premium gas with a booster, I bet that will get you your sticker.





Edited 4/27/2006 1:39 pm by danoman3 (danoman31)  

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omgang
V-6 Camaros rule!
Posts: 54
Joined: 12/04
Posted: 04/28/06 09:03 AM

I live only a couple hundered feet above sea level.  I did lean down my mixture screws and went through the test again and saw the CO levels really drop but the HC climb - exactly as you state, probably a lean mis-fire.


I have another post about high oil consumption and I'm really starting to wonder if the elevated HC counts are caused by the oil.  I've tried reducing initial timing significantly (down to 5 degrees) and the HC numbers improved a bit but not enough to pass - the engine ran like crap and was then too loud to pass sound levels as well.


Oregon allows a max of 300 for HC - the last time I went through my HC counts were in the 500's.  When I leaned the mixture out too far the HC counts went up near 1000.


I am going to check out the stem seals on the heads.  I don't know what type SD installs on their heads but I am hoping they're crap and a good positive seal will fix the oil consumption problem and let me get through emissions testing.


Thanks for the reply.


OM Gang

 

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danoman3
V-6 Camaros rule!
Posts: 28
Joined: 12/05
Posted: 06/02/06 04:17 PM

hello


 


Oh that was you with the oil problem, yeah that can definitely contribute to high HC. Did you fix that problem, if not then you will probably never be able to get the HC down. You could try thicker oil, but I doubt that that will help much.

 

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 06/09/06 02:35 PM

High oil consumption is telling you something isn't right with the engine and will show as high HCs at the emissions test. However, a lot of cam timing with a lot of overlap is also a large contributor. This is why late model engines with continous flow injection (TBI or TPI) use  short timing along with large LSAs to reduce the overlap. Sequential injection engines starting with the 92 LT1 can add duration and overlap while reducing LSA and get passed SMOG because the injector doesn't fuel till the exhaust valve closes so no unburnt fuel gets out during the cam's overlap phase. With a carb or continous flow injection you can't get away with much if any overlap.


So inquiring minds need to know about your cam's specs before going much further.


Bogie

 

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