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MG MGB Technical - Exhaust Gas Readings

MGB GT 1978 1.8l

Any one know what the exhaust gas readings should be.

Mine

CO 1.92 (%vol)

CO corr 2.66 (%vol)

CO2 8.90 (%vol)

HC 1107 (ppm vol)

O2 7.0 (%vol)

Lambda 1.297

These figures don't seem to tie up with anything that I have found on the internet.

Thanks in advance

Bob
R. C. Elwell

Hi Bob

Is your engine standard? The HC seems a little high.As long as the car passes the MOT does it matter what the exact readings are?



Peter
peter burgess

I replied on the other forum, but I'll repeat it here too:

"I'd guess too lean - CO reading should be near as possible to 4.5% (the MOT pass limit) and the HC much lower - normally around 200 - 400 ppm.

The lambda should be near to 1.0

Running lean will damage the engine in the long term as well as robbing you of power.

I also guess your spark plugs are white on the centre ceramic. Try turning the carb mixture screws clockwise half a turn. Ideally the plugs should be a light biscuit colour."

I am surprised at Peter's comment, yes I do think it matters, the high HC reading indicates excess unburnt fuel in the exhaust from the incomplete burning of a overlean mixture - not good for the environment or your pocket!

Remember though that these readings are only taken at idle - they will vary at other engine speeds and loads - which is why it is always a good idea to have the car set up on a rolling road - like Peter's - by someone who really knows what they are doing - like Peter!
Chris at Octarine Services

and here is a photo of what can happen in an engine run too lean - this one let go at 70 mph on a motorway ...


Chris at Octarine Services

Chris

The readings are at idle and as you well know totally at the mercy of idle speed,idle temperature under the bonnet,whether the cooling fans are on or off so the readings can go out the window...23 years ago I set up my first Rubber Bumper B on the rollers, it ran weak when it was hot at idle so I richened it to what you say...4.5% . The next day the car came back too rich as the mixture had autoleaned on the jets and I had over ridden the settings in my naievity! Set the carbs when just warm then don't mess, enough said?

You do not drive the car at idle, the most important settings are cruise and power. Rubber bumper Bs can be very awkward to get a nice smooth idle. You will even find this years batch of UK fuel seems to be making some rubber bumper Bs run a wee bit hot and bothered under the bonnet. The idle mix or one flat out would not burn an engine out on the motorway, using wrong needles and too much advance might well do it, but the idle mix wouldnt give a hoot!

Peter
peter burgess

Hi Pete,

I wasn't suggesting otherwise! Yes RB carbs are a pain as are the midget ones fitted with waxstats - the mixture wanders around with temperature.

I don't worry too much about idle mixture - the plug colour after a good run is probably more reliable! However an HC reading of 1107 is too high even if legal (just).

I tend to set carbs just a little richer than the ideal - otherwise I find that there can be a flat spot on pick up - my local MOT lets the cars through at 5% CO because they understand the issue.

And, yes, it was an air leak that caused the piston failure - but a lean mixture all the same.
Chris at Octarine Services

It's not an issue on my 73 roadster (smoke test only), but on my 75 V8 I have to weaken the carbs by 1/4 turn immediately before the MOT and richen it back again afterwards or they will fail it, and I don't want them fiddling with my carbs. If I leave it at the 'MOT pass' figure it picks up very poorly. This may be because it has tubular exhaust manifolds and K&N filters instead of the standard items, and I have also fitted 'richer' needles to get over flat-spotting when accelerating from a 65-70mph cruise. At the last MOT on my very erratic gunsons CO meter it measured 5.4% when adjusted for normal running and 2.4% weakened by 1/4 turn. The MOT report came back with 2.14%. HC can go up if it is running very weak, or any other causes of poor combustion.
Paul Hunt

Hi Paul

Which needles are you running?

Peter
peter burgess

Jeez Chris, solid sad evidence lean is mean! Vic
vem myers

Peter - they were the original BBU, and are currently BAK.
Paul Hunt

Hi Paul

Have a bash at BBC needles.

Peter
peter burgess

Quite a bit richer in the second half it is true, but they only seem to have 13 stations instead of the 16 for the others. Isn't that likely to cause a problem at wider throttle openings?
Paul Hunt

Paul,

No it does have 16 stations - it is just the last 4 are the same!

Actually I find the BDR is a better choice - it goes richer than the BAK and the BBC earlier but isn't as wildly rich as the BBC higher up.
Chris at Octarine Services

Hi All

Mostly the BBC or BCA seems to suit when I have run v8s on the rollers with SUs and K&Ns. Give the BBCs a try Paul and see what you reckon.

Peter
peter burgess

My poor choice of words, Chris, but the effect is the same - if you tried to use the last three stations the mixture would be all over the place. If it needed a mixture as rich as station 13 then wider throttle openings would surely make it very weak? having said that I don't know in practice how many stations are actually used.

I's be happier with the BDRs as it is progressive for the whole 16 stations as well as not so rich. However the BAKs have cured the original problem of mid-throttle stumbling i.e. weakness when smaller throttle response was good. The 'problem' now is that in order to pick up from idle smoothly the mixture needs to be richer than the MOT will allow. Going richer 1/3rd the way up isn't going to change that. I suppose what I need is an air-bleed valve that only opens when the throttle is shut :o)
Paul Hunt

Paul,

Fully up the piston will only get the needle to station 12 or 13 - probably not even that as I don't think the piston ever rises that high even on full throttle at max revs. Depends on the spring as well.

The BDR is richer at the bottom end so may actually allow you to weaken the mixture to MOT levels but retain the richer than BAK off idle.

Chris at Octarine Services

Hi Paul

Have you tried your needles on a rolling road?

Chris, ideally the piston should reach max lift at full throttle 75% max rpm.


Peter
peter burgess

Hi Paul

You didn't get back to me and say if you had rolling roaded your needles or do you monitor emissions while you are driving?

Peter
peter burgess

Hi Peter no to both, I only have access to a Gunson's Gastester which can be a bit variable in use. Other than that it is just 'seat of the pants', and apart from having to tweak the mixture for the MOT I'm happy with the car as it is. I'm a bit wary of rolling roads, at last where you start getting up to maximum outputs, heard some bad things but they could just be tales. I'm aware of in-car monitoring, but that is a bit 'G-force display' for me :o)

Some years ago there was talk of roadside emission testing, or even drive-by emission testing, but the former at least got a bad reputation as they were failing cars that hadn't fully warmed up and since then it has all been quiet on that front.
Paul Hunt

Hi Paul

Sorry to hear you are wary of rolling roads, they can be good tools in the right hands. I am sure a rolling road operator would comply with your request not to go over a certain rpm, bhp, throttle setting.

The alternative is something like an innovate AR ratio monitor which you could fit into your ex system, drive down the road and either monitor realtime or record data for later use.

I am not sure how you would know if your needles are right or wrong without some sort of testing? Weak is easy, pull the choke out and see if it goes better, can be difficult to drive whilst holding the choke :)

Peter
peter burgess

Pulling the choke when it was hesitating on mid-throttle acceleration was how I diagnosed the original needles as weak, it doesn't do that now.

One can do all sorts when the need arises, when my Lambretta broke a gear change cable near the gearbox I pulled it out of the inner and tied it round one ankle so I could change one way with the twist-grip and the other by a kick from my leg. All was well as long as I remembered to come to a halt leaning the correct way!
Paul Hunt

Thanks for that Peter - I did wonder if the piston ever made it up that far!

I just measured an HS4 carb throat and from jet to the top of the bore is 1.25 inches or 11 stations so the last 2 stations on a fixed needle and 5 stations on a biased needle never make it out of the jet.
Chris at Octarine Services

Peter & Chris-
Can't believe you guys don't know this!

I've lost my carefully done measurements, but the max piston travel is close to 1/2" less than the designated bore for all unmodified SUs. 1/4" for the bridge and near 1/4" for the piston sticking down when topped out. The dual restriction is what makes the variable venturi.

Max travel = 1 3/8" for -8 or 2” carb, nearly the same for -6, about 1" for a -4.

Add about 1 station for jet below bridge, so even a -8 only gets to #12. Remaining stations have a smoothing effect on fuel flow, but little change to mixture, since mixture control takes place at the narrowest point or topmost edge of the jet. There used to be short needles supplied for -1 & -2 carbs.

Max power is at max airflow is at piston high travel point. To get the most from the carb, the piston should be just short of top out at max power. If it tops sooner, the mixture goes awry; if later, you are wasting power sucking too hard on the restriction, and the mixture is harder to set since the active needle profile is essentially "compressed" lengthwise on the needle.

FRM
FR Millmore

FRM,

I agree the 1/4 inch for the bridge, but the pistons in my HS4s travel up to flush with the top of the bore.

I remember reading many years ago that the pistons never reached the top in practice but I have no idea where I read that or who the "authority" was!

I can't speak for Peter, but I am very much a practical engineer - I know what works and how to get the best out of things but I really don't care too much for anal introspection of the finer details! As a very busy person I just don't usually have the luxury of being able to spend time delving so deeply into things - but we have had this conversation before!
Chris at Octarine Services

Chris-
Indeed various carbs have different piston top out points, some H4 will go above the bore. So a bit of correction to my overly broad generalisation. What matters is what the one you are working on has. That's what determines how many stations you worry about.

As for "... anal introspection of the finer details! ", given that the selection of the spring/weight to get the correct piston travel is the FIRST parameter of SU carb tuning after the choice of carb itself, and the only one that affects the entire operating range, I always thought that "As a very busy person I just don't usually have the luxury of being able to spend time..." chasing my tail in circles to figure out what's going on whilst ignoring the basics. Don't figure you or Peter can waste the time either, which is why I expressed surprise above.

If one only does stock engines, then looking bits up in a catalog is passable, but as soon as you modify, it gets dicey. Given that most mods are expressly to increase airflow, and that this may cause piston top out, and that it is known that SUs go lean when the piston tops out, it seems critical to me, and I would think to you - or was the ugly scene above (post #4) posted by an impostor?

Further, since the piston position determines the station in effect at a given time, you cannot usefully discuss needle dimensions until you know this. The commonly shown "ranges" of needle effect are only generally representative of more or less standard engines.

FRM
FR Millmore

FRM,

The mods we carry out on engines are well proven over many years and the needle & spring choices are well documented and backed up by experience on the rolling road. Reinvestigating these would just be a waste of time.

Very few of my engines, even the modded ones are used at the extreme of performance, the vast majority of customers use their cars gently!

Those who do use their engines in anger report excellent performance and one of my race engines has actually won recently at Les Mans - so I must be doing something right!

Oh - and the destroyed engine wasn't one of mine!
Chris at Octarine Services

Hi FRM

I am looking for max lift at 3/4 rpm, which I suppose is between peak torque and peak bhp. That is what I was taught by a couple of people who worked at SU and came to me with cars to tune and passed on some of their knowledge to me.

Do you run a rolling road or dyno for research?

Peter
peter burgess

This thread was discussed between 24/06/2009 and 24/07/2009

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