MOT fail for CO2 - adjusting a Stromberg 175 CDUS
 

Hi all,

I have a 1978 245 fitted with a Stromberg 175 CDUS which has failed the MOT as CO2 levels were too high. I haven't done anything to the carb yet, so a starting point is to strip it down, clean it and put it back together.

I can see from receipts from the previous owner that a new jet and needle were fitted in 2011, which equates to about 55,000 miles ago based on MOT mileages. I guess they're due for replacement then - although I'll look at them first to see if there's any obvious damage.

I also have a complete carb from another car which had 90k on it, but I have no idea how good it is or whether the jet and needle in that were ever replaced etc.

My question then - how do I/the garage go about adjusting the carb? I don't have a co2 meter so it'll have to be the garage realistically. I've seen a few threads online, most usefully this one:

https://www.volvoforums.org.uk/showthread.php?t=100472

But I'm not sure if my carb is an earlier or later type, as referred to in that thread.

It looks like this:

https://imagizer.imageshack.com/v2/1...922/6PMvCv.jpg

Any comments/help/advice? From the thread above I think if the dashpot is removed the needle can be raised up and down to adjust the mixture via a special tool, is that correct?

Thanks

There is a brass 8mm nut on the underside to adjust the needle , what was the measured CO at idle ? These carbs were notorious for going rich due to several reasons , The needle and jet wore after about 25000 miles , unleaded fuel is unkind to them , the float
level would rise due to the plastic bridge piece holding the float hinge weakening , also there is a silver valve on the side with a U shaped rubber pipe on , these fail and you get massive richness on choke … they were like this way back when 240's were current , let alone now some 25 years later !

You need to replace at least the above parts and a float chamber gasket and diaphragm for good luck

Hi,

Thanks - hmmm, doesn't sounds like it'll be much fun messing about with that carb then. I don't know what the measured co2 at idle was, the wording on the failure sheet is:

Exhaust emissions carbon monoxide content excessive (8.2.1.2 (b))

Which I believe refers to the fast idle test.

I wonder if the other carb is any better. Unfortunately without an emissions tester it's hard to tell without frequent trips to the garage.

Thanks

you are going to have to spend Money what ever you do , best get the current carb overhauled and set up just the once to pass . It should make a big difference to fuel consumption too and sweeter running ..

It would be worth asking the garage for the detailed emission readings, at idle and at fast test. Normally they give you a separate print out, apart from the certificate or failure notification.

(By the way, it's carbon monoxide CO, not dioxide CO2. CO2 is the complete combustion product if the settings are correct, taking the CO down to the acceptable level)

Ok, I’ve dug out the paperwork and there is a specific figure given- the co (not co2, thank you) reading was 6.09% and the limit is 4.5%. There was only one set of figures given, I presume for a car this old it’s only tested at fast idle?

Anyway, since that’s significantly over the limit I imagine anything short of sending the carb away to someone who knows what they’re doing with it would be fairly futile...

So I’m going to swap it completely. I’ve got a manifold fitted with a Weber 22/34 dmtl that I bought from a forum member a little while back, it was on his b21 equipped car and was set up properly on a rolling road so hopefully it’ll bolt on and work with minimal fuss.

If I didn’t have the weber sitting there I’d put more effort into fixing the Stromberg, but in the interests of time - optimistically booked mot on Friday, car show Saturday - I’ll swap the Weber in and hope that works.

Also- from the looks of this thread others have been happy with Weber conversions to redblock engines...

https://www.volvoforums.org.uk/showthread.php?t=155444

Cheers

6.09 isn't miles away from 4.5. I'd be tempted just to turn the mixture adjuster gradually in the weaker direction for as far as the engine keeps running nicely, and then try again. It may just be the previous owner fiddling with the setting.
You've nothing to lose as you've got a replacement carb anyway.

Was the dashpot filled with oil, is your air filter new, also check the diaphram for any splits, easiest things to check first.
I would also check plugs, points and leads, distributer cap and HT leads as well as checking the timing is spot on.

P.S looking at your pics, it makes me miss my old 1979 240 DL,

Thanks all,

I suppose before going to the trouble of changing the carb I could look for obvious faults. My hesitancy is that my self imposted limit of the mot on Friday afternoon is looming closer and I won’t be taking this car to retro rides on Saturday if it fails. Of course fitting the weber carb may not be trouble free either, but they’re a much simpler carb with an idle mixture screw that works - my impression, perhaps unfairly is that the idle adjustment of the Stromberg is temperamental at best. If I found an obvious problem with the Stromberg that I could fix (perhaps with parts from my spare Stromberg carb) then I suppose I’d be more inclined to risk the mot with this carb fitted.

The distributor cap, rotor, spark plugs and leads are fairly new and the car runs well so I’m not pointing a finger at them yet. The dizzy is the original one with points in it, I have all the parts ready to swap to a later dizzy with a hall sensor and ignition amplifier, which I’m planning to fit before the mot as the only time the car has run poorly was because of points - and although I can fit them to spec I don’t know enough to start tinkering with gaps etc.

The air filter isn’t all that new, I’ll take it out and blow it out as best I can.

The timing I’ll set when infit the ‘new’ dizzy- remind me, was it 25degrees at a fast idle? I’ll have to check.

Dashpot oil... haven’t checked that. I believe I should take the dashpot out of the top of the carb and then fill with motor oil to... where?

Thanks,

Cheers

Ok, went and tinkered with the spare carb in the garage. The diaphragm is complete with no splits, as is the float bowl gasket. The little silver valve thing on the side appears to work, in that I took it off and if I blow into it there’s no airflow but if I suck on the valve it opens and lets air through. I can see how the operation of that valve would be massively affected by the condition of the hose that connects onto it- the hose on this carb is perished at both ends. I tested the float and it holds air and doesn’t leak.

I have a tool that fits the mixture adjustment on the bottom, but I don’t have a tool that will fit the... not sure what it’s called, the float bowl inlet jet in the float bowl that keeps the float bowl full but not overfull. It’d have to be an extremely thin walled socket to remove it as it’s close to the edge of the floatbowl.

The main needle is clearly bent, so I’m curious how the car this came from ran...

I think I have enough spares to at least remove the current carb and look at it.

Cheers,

So- dashpot, how much oil?

And is there a guide online anywhere to tuning these carbs?