MOT Fail - CO Emissions

[josh2109]

Hi folks,

My Scirocco failed it's MOT today :( It failed on the carbon monoxide content of the exhaust gasses, and the linkage on the nearside front anti roll bar is mullered. The tester says the bracket has come off completely

So, need to fix her in 10 working days, which leads me onto my question RE the CO reading:

Is there any way to set up the carb for the correct CO % without using a meter? I dimly remember someone on a Golf forum saying the CO screw on the 2E2 should be wound all the way in, then 2 turns out for the correct setting as a rule of thumb. Is this right? I set mine up by the revs it idled at as I turned the screw, but thinking about it I reckon I should've wound the screw in, and sorted my idle out using the adjustment on the 3 point unit, then adjusted the CO screw... I'm losing track of my thoughts now, so I guess what I'm saying is what's the best procedure to set up my CO by without a meter.

Cheers,
Josh

[james butler]

Wind the screw in counting how many times you turn it until the engine starts to falter and then put it in for retest. When it passes turn it back to where it was. Then you have another year to get it properly looked at.
I did manage to tune my roco using a gunsons colour tune kit which worked ok. But you really need to use a emissions tester to be sure it's ok to pass an mot

[fonzooorooo]

It' s enough of a faff to set up WITH a meter(!) ... you have to continually balance the mixture and idle screws... If you just set the mixture, then crank the idle down, the CO alters wildly!

Saying that, it's only 10 mins and 2 screws. You want 1.6% on these. If nobody near you's got a meter, I'd ask if the test centre'll sort it for you.

Outer front ARB brackets are a nice easy DIY job to make if you can't get hold of any!

[Nate]

how far out was the CO reading? had the tester made sure the engine was fully up to temp and the autochoke had come off?

[josh2109]

It was 5.9% No idea how hot it was, maybe I'll rag it around this time before handing it over

[Nate]

It was 5.9% No idea how hot it was, maybe I'll rag it around this time before handing it over

what was the HC reading?

[josh2109]

It was 5.9% No idea how hot it was, maybe I'll rag it around this time before handing it over

what was the HC reading?

The hydrocarbons were at 494 parts per million, well below the 1200 PPM limit.

There's no other way to adjust the CO on these cars other than the screw on top of the carb, is there?

[Nate]

on a 2E2, you need to be 100% certain that its coming off choke. if the choke ain't coming off the CO will be very high (as yours is) also make sure the air filter is clean (replace if in any doubt, they ain't expensive) as that will have the same effect. i'd wager it would be more likely to be one or both of these, rather than the carb mysteriously wandering a mile out of tune since the last test

[josh2109]

Ah, thing is this car isn't fitted with the carb it had it's last MOT with I hated the manual choke and have gone with a spare 2E2 I had. Will swap the CO screw housing from the old carb over to this one, see if that sets it back up the same. Will ask the MOT testers if I can check the choke is fully open before they take the car in too! I have a spare clean filter somewhere so will pilfer that for the MOT and hopefully all will be well :)

[Nate]

running that rich, you would expect to see piss poor fuel economy and probably poor performance too. i'd suggest that having made sure the air filter is clean, just get the tester to set it up for you while they got it on the gas analyzer

[josh2109]

Is the CO % a direct reflection of how rich or lean the idle mixture is then?

I went and checked the filter today, the one fitted looks brand new, no dirt on it whatsoever. The choke is opening quite quickly too, and stopping in the full open position. It seems to open up quicker than the choke on my Golf does. I let the car get up to temp anyway, and the choke remained fully open, so I've no reason to suspect that it had it's emmissions tested with the choke closed. seeing as these two alleyways seem to be dead ends, I just went nuclear with carb cleaner I removed the idle screw & housing, then cleaned both thoroughly. I sprayed cleaner through the idle jet too, and over the locations of the other jets. After putting everything back together, I can now wind the screw in all the way without the car cutting out, whereas before it would start spluttering and then die several turns before this. At any rate, I've turned the screw out a couple of turns like I did on my Golf. I'm hoping this is enough, but if not I'll just get the tester to sort it - i can't spend my whole life tweaking and retesting it

[Nate]

Is the CO % a direct reflection of how rich or lean the idle mixture is then?

more or less. the richer the mix is, the less air there is for the fuel to burn with, so you will get more incomplete combustion (which produces CO instead of CO2)

[josh2109]

Is the CO % a direct reflection of how rich or lean the idle mixture is then?

more or less. the richer the mix is, the less air there is for the fuel to burn with, so you will get more incomplete combustion (which produces CO instead of CO2)

Ah, I see, that makes sense :) I wonder if the carb cleaner may have helped a little then - I think the idle fuel/air jet was a bit clogged up, as I have far more useable adjustment in it now. I should've cleaned the carb before I fitted it really, but as ever I was rushing

Is there any hard and fast rule as to what effect turning the CO screw has on the CO % of the emissions? I see that it operates by enlarging or shrinking an aperture in the side of the screw housing which in turn alters the volume of air able to enter the idle fuel/air jet. With my Golf I was lead to believe that the further in the screw was, the better. I'm thinking though that more air entering the jet will make the mix more lean, but at the same time I find that allowing more air in raises the idle RPM and I'm thinking this will start drawing more fuel from the carb and raise the emissions as a result Argh this is horribly complicated without a probe!

[Nate]

you'll also find you can further inprove the running of it by setting the ignition timing right. you can either do it the boring way as per haynes using a timing light, or just keep advancing it until it starts to pink under load, then back it off slightly. you might find it's worth investing in a "colortune" i've had a reasonably amount of success setting up cars with this in the past. it works by setting it up so the combustion is optimum colour, which should give you the right emission level out the back

[josh2109]

you'll also find you can further inprove the running of it by setting the ignition timing right. you can either do it the boring way as per haynes using a timing light, or just keep advancing it until it starts to pink under load, then back it off slightly. you might find it's worth investing in a "colortune" i've had a reasonably amount of success setting up cars with this in the past. it works by setting it up so the combustion is optimum colour, which should give you the right emission level out the back

I have a timing light lurking in my tool cupboard, so may break that out and take a bash at the timing. I was going to check it with the light not long after I bought the car, but the hassle of removing the metal cover put me off (I'm a bit lazy at times ), I was execting a clip off plastic cover like my Mk2 Golf has. Although when I timed up the Golf after the addition of a GTi cam I got bored of the gun, so filled up with a tank of V-Power, grabed a 13mm spanner and ran up and down a quiet stretch of road then adjusted the dizzy each time I stopped. Advanced it to where it pinked, then backed off a tad and all was well :D until the day I ran out of petrol too far away from a Shell station, and had to visit Texaco and bought some of their regular stuff

Thank you for the tip RE the colortune btw, I've been after something like this for ages, but didn't know what to look for!! :D I've reserved one at my local Halfrauds, so will grab that tomorrow and get my fuel mixture set up as good as I can get it. What with having 2 cars on carbs it seems like a good addition to my current tools at any rate. Especially as I have some bike carbs sitting in the loft