Running a 383 with AFR heads and about 10.3:1 compression. Got a solid roller Magnum cam (294S) that's 248/248 @ .050, and .525/.525 lift and 1.65 rockers for about .575 lift. Running a Demon 750 double pumper

The first time I ran it I ended up failing HC and CO - with / NOx
CO was about 8 (.3 is passing)

After increasing the lash (26/28), bumping up the timing (42 total) and leaning out the carb, I passed the HC, but still failed the CO with a score of about 3.8

Third time's a charm right? well... not for me. I leaned it out even further, down to 66 primary jets, and ran a 2.5 power valve to make sure that was not an issue. Driving the thing to the shop was tough - it was so lean it made no power above idle. Still failed with CO of about 3.0


So to sum up.... it was too rich - was still too rich - and was then STILL too rich. I just don't understand as the thing is so lean it barely runs. What else can I do? I'm not looking for "register it elsewhere" comments, because I am already thinking about it. This is more a question of "what the hell is going on here"

 

Sounds like typical "too much cam" for emissions.

I'm assuming that your AIR is in good working order and that you have a new high-flow cat installed? If you don't, you don't have a chance.

That sure doesn't look like a solid roller to me; looks like a flat tappet. Not that it really matters a whole lot, but it's good to be accurate about what you've got. You might get answers that don't really apply, otherwise.

Put the lash back where it belongs (.020"/.022" IIRC), re-calibrate the carb to where the motor runs the best, by decreasing the jets until the car just barely starts to surge at highway cruise, then go back up about 2 sizes. Make sure you have the secondary idle set properly so that the primary throttle blades are in the correct orientation with respect to the primary transfer slot; which is where there's about .030" - .050" of transfer slot showing below the throttle blades when at the correct idle speed.

Then go from there.

 

You are right - sometimes I write without thinking. It's a solid flat-tappet cam.

There is no AIR setup, but it was my understanding that the AIR only affects the NOx reading - which in my case is not a problem at all. Does it also affect CO?

What you suggested as far as recalibrating it to run well is exactly how the car was the first time I brought it in.

I think your first statement may indeed be my problem - just too much cam to hope to pass emissions.

 

That is erroneous.

The purpose of AIR is to introduce fresh air into the exhaust stream, in order to complete the combustion process.

The purpose of the cat is to combine whatever remaining unburned combustion products may remain with oxygen. It can't miracle up some oxygen out of nowhere. There either has to be extra oxygen left in the exhaust itself, which requires EXTREMELY lean operation to attain; or it has to be injected into the exhaust downstream from the engine.

It is possible to get some amazingly improbable things to pass emissions, once you (a) understand how those things actually work, and (b) put your car back together without hacking off all the devices that were necessary to achieve compliance even when the car was bone-stock. It doesn't make alot of sense to leave the "accessories" off, then modify the engine itself so that has no possibility of complying, then wonder where you went wrong.

Start with those 2 things, see where that takes you. Then put your carb back like it came, and learn about how carbs work; and re-calibrate it if necessary.

I'm betting that it will be possible to get it to pass, once you get past whatever misconceptions you have, and you learn the realities of the systems and how to use them to your advantage.

 

Well I appreciate your input. I did a pretty good job of tuning and lowering all of the bad exhaust values given the lack of ANY emissions equipment other than a cat. I think I've got a very good handle on how to tweak a carbureter.

My only misconception thus far is that I didn't realize the AIR system would help my CO numbers. Now that you mention it, it does make sense. Perhaps that's my only problem at this point.

This car was not built to pass emissions - not by a long shot. I'm sure you realized that just by looking at the size of the cam. At this point passing emissions would be more of a novelty than anything else - and my asking questions here is to learn exactly what you just told me. I picked up as much as I could while searching, but it never works quite as well as asking a direct question.

I wonder if a stock LS1 style electric smog pump will give me "enough" oxygen to pass. I could throw that on temporarily and give it another whirl. Does it sound reasonable that that's all I'm missing?

 

They hardly ever are. But then, reality sets in, and we decide we'd like to DRIVE these cars we've got all our money tied up in, instead of just admiring it as a garage ornament; and we've got to get practical all of a sudden.

I couldn't say for sure about the LS1 pump. It seems a reasonable approach. If it's easy to try, then go for it. Basically anything you can do to get some O2 in there, so that your cat can do its job, will at least help. The cat's job, as described above, is to combine those last few molecules of incompletely burned fuel, and CO, free oxygen, to make CO2. Try to get plenty of air into both the header primaries (to actually ignite the not-quite-spent fuel), and the cat's air port (to give the cat some oxygen to work with).

Whatever you can do in the way of carb calibration and ignition setup (timing) to promote COMPLETE combustion, will help as well. That usually means advancing the timing CONSIDERABLY more than stock, and getting the mixture close to stoich. Ignition timing is widely misunderstood as well; people, especially those used to FI, are accustomed to this notion that you look it up in a book somewhere based on the year and model of car, and set it there, and it's then perfect. Nothing could be farther from the truth. Generally, the more cam you have, the more advance you'll need at idle; but the high-RPM requirements don't change much. For a typical SBC, it will always need to be somewhere around 34-36° of "total" timing (not including vacuum advance). But a stock dist is set up to give 20-24° of advance, which means that you're going to have to start out with only 10-14° at idle, which often isn't enough. If you can change your dist to get about 14-16° of change out of the spring/weight system, and set the "initial" timing to 20-22° or so, you can often get FAR better results; in driveability as well as emissions.

Too much timing however will make the combustion temp and pressure go too high, and you'll get too much NOx as a result of that. Compression and timing advance are the primary culprits for that. So if that's in tolerance in your car, you have some "wiggle room" to help out those other things with.

 

I understand it can't miracle up anything. In a perfect system, the NOx to N2 + O2 reaction happens in the front part of the cat, and that O2 can be used for the CO + O2 to CO2 reaction - in the back of the cat. I'm getting zero NOx, and very low HC's. All indications point towards it still just being too damn rich. If all is well, and you are running right at stoic, you should not need the AIR injection to have clean exhaust.

"This is because the three-way converter works best when the air-fuel ratio of the engine is kept within a certain very narrow range of the 14.7:1 Oxygen:fuel stoichiometry point. Within that band, conversions are very high, sometimes approaching the theoretical point of perfection (i.e. 100%)"


I agree with you that this thing *should* be able to pass. The fact that HCs are low shows that even with the big overlap, everything's getting burned. So it seems like it's got enough timing, I just need some more O2 in there to clean it up. I suppose if I drove it even further to the lean side I should be creating more NOx which would provide more O2 in the cat, and help out the CO. I'm already down to 66 jets up front and it runs terrible (no power at all) I doubt I can lean it any more and retain any driveability.

Unfortunately, I do not have AIR headers. I'm starting to understand why they built the manifolds like that from the factory. I wonder if pumping air to the cat will be enough. I need to reduce my CO from 3 ppm to 0.3. That seems like a big deal - a factor of 10.

 

You've confused the need to lean out the idle and low speed cruise circuit with the main circuit (main jet). Fatten up the main jet till the driveability comes back. Then reduce the idle feed restrictions in the metering blocks.

At idle and at the engine rpm of the 2525 part throttle test (25mph) the carb is not on the main jet yet. The fuel is controled by the idle feed restriction during this test. You need to fine tune the idle feed restriction. possibily the idle air bleed too.
The tempeture of the intake manifold will affect the apparent AFR as seen by the running motor, a lot. Don;t try to tune it or evaluate the tune until the motor is fully warmed up.
CO is directly tied to AFR so when the AFR is correct the CO will be low.

 

I would go the other way with the timing. Retard it to raise the cylinder temps. for a more complete burn......

 

retarding timing will not raise combustion chamber temps. when you retard the timing it doesn't creat the pressure which creates part of the temps inside the combustion hamber itself. retarding it will instead raise the exhaust temps though as the mixture is still burning on the way out the exhuast. waste of power