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Failed Emission Test


dss LT1

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My 72 LT-1 recently failed the AZ emissions test. It passed under load and failed at idle. It has a new MSD ignition system that is set at 36 degrees total advance (before mech adv).

I am trying to fine tune the carb (original Holley, recently restored) to lean out the idle and balance that off with maintaining a reasonable idle speed and mixture. I am also not running the original AIR pump (not sure that makes a big difference).

I have read some of the tuning posts on CF. I have adjusted the mix screws on either side of the front of the carb - about (a little more than) one turn out from lightly seated. How should the accelerator pump be adjusted? I believe that if it is set to aggressively it could be adding fuel at idle... not sure how I would know if it is set properly.

Any other suggestions from the crowd?

Dave

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Dave it's been awhile since I've messed with a carb, but from what I remember the accelerator pump just gives you more gas when accelerating?

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Dave, we have trouble every year with our carbs as well. They are a bear to get tuned and frankly, I'm thrilled that we won't have to with the new emissions law. Any reason you're not waiting until you can exempt the car?

ps-if you find a shop that can actually tune a carb for emissions, let me know-I've had nothing but bad luck with any shop I've tried.

M~

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Maybe I am messing with something I shouldn't... here is what I just found on http://www.bob2000.com/carb.htm

"Accelerator Pump arm adjustment: The accelerator pump arm must not be loose at idle. The .015 clearance is measured at wide open throttle (engine off!). This is to prevent damage to the pump arm and diaphragm. You may need to adjust the external spring one way or the other to get it tight at idle, yet still have the .015 clearance at WOT."

So maybe I have part of the answer.

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My plates expire in Feb, I need a clean emission test to get a renewal. Not sure when this new law takes effect. Doesn't it also restrict mileage and require collector insurance? I am not cool with restrictions on how/when I can drive the car.

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Idle adjumstment screws are where you need to be.

Basic parameters of idle adjust are lean 'em until the motor starts to quit, then back 'em out 1/4 - 1/2 turn.

Timing: you can retard the timing a little also ..makes it burn a little hotter at idle, and idles a bit smoother.

-Frank (a Holly guy for years)

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:agree

You are missing the smog pump, so the extra O2 that was pumped into the exhaust manifold to burn off the unburned hydrocarbons is missing at idle!!

You did the same as pulling or gutting the cats on a new Vette!!

Ken

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Dave,

What are the numbers of the emmisions that is failed by? Just wondering how close it was.....

Andrew

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Prolly shuuda mentioned ..

.. accelerator pump arm on a holly

.. adjustment has to do with the arm binding at wide open throttle

..you bend the soft metal 'wire' arm to get the adjustment clearance needed tween the lever and the pad that pushes on it.

You don't want any clearance at all when it's at idle because if there is any slack in the linkage at all .. you got bog as the butterflies open up due to no gas bein there to mix with the initial increase in air into the motor.

You can adjust how the pump works by changin the plastic pad (or cam) it runs on ..they useda make a ton of different ones all in different colors and the trick was to get all the pump had to offer in one smooth shot.

The cams would also change the amount of gas discharged as the butterflies opened, ie: a bigger/smaller shot initially tapering or building up as the throttle opened. You can literally tune out any bog you have ..they even offer a 50cc pump if you need more than the 'stock' Holly pump can give.

(it begins to look like a toilet flushing when ya punch it :)

-Frank

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My plates expire in Feb, I need a clean emission test to get a renewal. Not sure when this new law takes effect. Doesn't it also restrict mileage and require collector insurance? I am not cool with restrictions on how/when I can drive the car.

IIRC, it's 1 March when the new regs take affect. My AK tags are good til '08, so I'm ok. I get no where near my mileage limit on my insurance...I think you'll be fine.

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My plates expire in Feb, I need a clean emission test to get a renewal. Not sure when this new law takes effect. Doesn't it also restrict mileage and require collector insurance? I am not cool with restrictions on how/when I can drive the car.

Yes, the new law does require collector car insurance. The law recognizes that most collector insurance limits mileage, so that is not written into the law. In addition, depending on which insurer you go with, they can be very lenient about mileage and type of driving. Mine doesn't ask for an update on miles (so they don't know how much I've driven in the past year), and it's okay for me to joyride or drive to work occassionally (ok, like once or twice a week). I'm assuming your vette is not a daily, so you'd likely be okay-the only other stipulation is that the car is garaged nightly. I'll take a wild guess that you do that also.

To somewhat go off topic-does anyone know if Dave would need to be registered on current rules, and wait until next year for the exemption, or could he just pay his one month late (I think it's $8 late fee-less than the test anyhow) and get registered? that would be the only hitch I could see -

M~

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Failed at Idle, passed at Load

Idle:

HC standard: 400 ppm, my car: 491 ppm

CO standard: 5.00%, my car 7.85%

I believe these are AZ standards but not too sure.

I am running the original Holley 6239 carb, it was just restored by Chicago Corvette. Sorry but in the earlier post I meant 36 degrees before vacuum advance. I believe the initial is 12 degrees, gonna check it again this weekend. I am also cleaning up the smog pump and may put it back on.

It is not a DD, I like to drive it once or twice a week to work (25 mi RT). Currently I have the car listed as my 3rd vehicle with 21st Century Insurance. I'd probably be able to meet the minimum mileage but all of the collector car insurance I have seen forbids driving to work, so I went elsewhere.

Thanks for the help!

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Like Melissa said, we always have this problem with Imaginary. (I guess that name dosn't apply anymore...) I usually put new plugs, one hotter than stock. Adjust the mixture screw around 3/4 to 1 full turn. If the car idles at 900, I usually raise the idle to around 1100-1200 rpms. So far, that has made it pass. Unfortunately the timing is something that I do by ear (old school is the only way I guess...) because I have yet to have it run right when I use a timing light.

If you have a bigger than stock cam, then the idle will need to be raised otherwise with a lower idle it won't burn all the fuel.

Good luck.....

Andrew

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  • 3 weeks later...

All right - Thanks to all of your assistance - I passed.

I reset the timing to 10 initial, I am running a MSD HEI.

I backed the idle mix down until it caused a 25 rpm drop on each side, then re-adjusted the idle to about 925.

Here are today's results:

Loaded HC: 85 (was 104)

Loaded CO: 0.24 (was 1.17)

Idle HC: 214 (was 491 - fail)

Idle CO: 2.83 (was 7.85 - fail)

Good to go for another year!

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*nice* goin :)

Imagine my trial with the smog station guys runnin a 6-pac 440 powered Duster thru ...509 lift purple shaft hydraulic cam, 10" converter back then .. I got it close .. but it just would not match the readout of the original 340 that had been in the car -evil grin- .. I got my plates anyway ..

..seems Chrysler applied for and got a smog exemption for the 1971 cars.

If they are not obviously pumpin smoke out like a mosquito fogger, you can get em thru emissions :) Just gotta have the vapor canister on the fuel tank vent, and PCV valve with hose into the air cleaner :)

-Frank

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  • 3 weeks later...

Not yet. But Melissa is gonna register Imaginary under the new law.

Andrew

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  • 2 weeks later...
anmracing2: I would like to know how the registration went, what were the problems. I'm looking to try in June.

Brad

Sorry-ours isn't due until July, so I guess someone else gets to the be guinea pig....

M~

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