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MG MGB GT V8 Factory Originals Technical - On board exhaust gas analyser

I have converted MGB with a 3.5l Rover engine and copied the factory carburateurs being dual SU's.
When rebuilding the engine i polished the cilinderhead, fitted a mild road camshaft and K&N filters. I also bought new needles for the carburateur after explaining the supplier what i have done with the engine. I now have the feeling that the the mixture is still a bit lean. I have the feeling that the car is driving oke and the coolant temperature gauge is even with hot weather in the middle of the scale.
But the spark plug show a light brown color and I simply cant adjust the carburateurs in a way that the spark plug are getting black. Now i am considering to get a exhaust gas analyser which can be fitted to the car. It is a device with a box with green, yellow and red LED's and a lambda sensor which has to be fitted in the Y-exhaust joint. In this way one can see, while driving, if the mixture is oke, lean or rich.
The price is reasonable, being GBP 90,- (US$ 120,-). The brand is Lumention.
I wonder if anybody has any experience with such a device.
The alternative is off course a rolling road but this is more expensive and one need to pay again for next times.

Peter van de Velde

They work pretty well, certainly get you in the ballpark. Not a bad idea in cars that are essentially custom. GPB 90 is a ripoff unless it comes with the o2 sensor. An exhaust temperature analyzer is better but considerably more expensive.

I like the one that Simple Digital Injecton sells. You can look at it on their site

I think it is around $70. Jegs or Summit Racing have them for around $30.


Correct me if I'm wrong but brown is the sign of a correct mixture (along with gray with some fuels). A black and sooty plug is the sign of an overich mixture. Black & Oily engine problems. White/Glazed clean looking - weak.

May I would suggest you purchase a couple of different needle profiles and take your car to a rolling road and have them assess the mix using with a wide band O2 sensor & measuring equipment (normally a probe put in the exhaust). They also generally measure CO emissions at the same time. If it does need altering rich you'll be ready with the needles. Search on the web for suneedle.exe and you'll find an app that'll let you work out the next 2 richer needles than the one you're currently running.

AFAIK the cheap gauges and sensors don't give much info except either side of Stoic (14.7:1). To tune properly you really need to know 11:1-18:1 +/- and the cheap O2 sensors don't do that. ie at WOT you would like to see maybe 12.5:1 for full power applications. The issue is the sensitivity of the O2 sensor. You have to spend around $1000 +/- to get something really worthwhile and a lot of that is in the sensor cost. Solutions untilising the Bosch Wide Band sensor generally offer the cheapest solutions but I haven't seen one for less than $900. A rolling road session would be much cheaper if you're only going to use it to set up one engine. The cheap sensors will _work, if only to tell you you're rich or lean - you just won't know with any accuracy by how much - no matter how many flashing LED's the gauge offers - it's all in the sensor.

HTH! :)

Neil Cotty

Ped, thanks for pointing me into that direction. Interesting site.

You seem to know more about sensors ect. I agree with you that brown is generally the correct color for the spark plugs but my plugs are very light brown (caramel kind of color). What confuses me is that i simply cannot adjust the carburateurs in a way that the spark plugs become sooty black. In the meantime, i did some further checking and adjusted the float chamber level a bit higher. I also have another pair of SU's and these had lighter springs on the pistons and i fitted these. It seems to have improved acceleration a bit because my idea was that especially with acceleration the mixture was too lean.
I agree with you that a rolling road is the best but we don't have so many good ones in holland who have a collection of needles and knowlegde/interest in classic cars. The one that has is 2.5 hours drive and charges 250-300GBP. You are also not allowed to stay with the car during the session.
For repeating checking this on board exhaust analyser looked like a good alternative but your comments offer a great deal of good information.
I work with a compagny which manufactures lifttrucks and we have a supplier who sells 3 way ctalytic set-ups to us. Maybe he can help me out with a good sensors or analyser at a sensible price. I will keep you posted.
Peter van de Velde

This thread was discussed between 26/05/2001 and 30/05/2001

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