FueL Air ratio confusion

My 4.2 l engine for 1968 etype has been rebuilt with new rings and valves. Compression is verg good and about equal on all cylinders. Static timing is close to 10 degree btdc. Now trying to adjust two zenith stromberg carbs and finding very conflicting info on Air fuel ratio.
On one hand, the exhaust has a little smoke and smell which I assume would mean a very rich carb mix

I also have an exhaust tester which measures oxygen and reads out in air fuel mix. I have tested thiS device on another good car and get an afr of around 14.7 which is correct

But on my Jag, I get a very high reading of over 20 which means a very lean carb mix

Is this conflicting data possible or am I doing something wrong?

Incidentally screwing the carb mix screw in or out doesnt chane the engine performance much and always have apparent conflict of exhaust smoke and high afr readings

Any thoughts focused on this seeming conflict in info?

I’m not familiar with ZS carbs. On SU’s it’s easy to visually confirm that the mixture control - the rise and fall of the jet - is operating (per screw) correctly. Are you able to confirm something similar for yor carbs, before you look further?
Also if the car has new rings there might be some oil smoke present before they bed in - usually accompanied with an oily smell. Paul.

He can do the same piston lift test.
Lift the pistons one by one by 1-2 mm to make the mixture leaner, if rpm rises it’s rich, if it falls immediately it’s lean, if almost nothing (rise, then slight fall) it’s correct and if it runs the same no matter what it’s not working aka really lean.
He’s looking for less than 14.7 but he has to balance the carbs first etc. and also check both tail pipes.

The Strombergs are not blessed with easy or good adjustments in the adjustment department. One or both of mine were running lean. Pressing the jets up and down to change fuel flow is carb-out repetitive work - even then David and I couldn‘t make it run nice. I bought adjustable jets and am happy with result - runs great. Thanks David :wink:


No one has focused on my question of how I can apparently have conflicting evidence of rich carb mixture (smoke, smell) and very lean mix (vrry high air fuel ratio) at the sam time.

Has anyone ever observed this???

Here’s a thought. Since you are apparently using a sniffer in the tailpipe it’s possible that you have an exhaust leak somewhere down the line that is pulling in additional air skewing the AFM readings by the time it gets to the tailpipe.


I presume you are using an Innovate or similar wideband meter. I have found that these can give very strange readings on twin exhaust cars if used with a probe in the tailpipe - a leak is one possibility, but I have found bizarre readings if there’s no balance pipe, for example. I weld a bung near the collector on one or both sides to take a threaded λ sensor as outlined in the manual - the tailpipe probe is not always reliable.
Failing that, a brief rolling road session would be a good idea.

Roger. Thanks. Yes I have an Innovate meter and there is a connection upstream on two pipes so same mix should be in both tail pipes.

But very bizarre when black smoke coming out and meter reads 20+ afr. How is that possible?

It’s odd - I had this problem on my 289 Cobra, and the only fix I could find was fixing a bung near the collector. There were no leaks that I could find - it was as if air were sucking back into one pipe showing a weak mixture.

You seem to be blindly trusting technology.

If it is super rich, then it may not read reliably and simply read off the scale and so “20” may be “meaningless” rather than “lean”. Remember it is measuring spare oxygen and so if you can pull air into the exhaust, it’ll give a lean reading regardless of degree of combustion.

If it really is that lean, your spark plugs will be very pale and there may be pinging noise in the inlet if ignition is overadvanced.

kind regards

have you tried useing “colortune” that way you can see in the combustion chambers which will give you a more accurate visual.

I don’t use my wideband O2 meter for absolute settings - those are done on a rolling road. I rig it up for temporary road use, so I can check for any changes in AF ratio under acceleration and load to give me a picture of the carb settings. I wouldn’t trust it for definitive initial setup AFR readings.

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After gathering more data, my guess to my own question is that one of two carbs is running a little rich producing smoke and smell and other carb and three cylinders are operating very lean, giving an overall lean air fuel ratio in exhaust

Does this make sense?

I said to check both tailpipes but you have a 2+2 so I think on these it doesn’t matter since they mix. Yes makes sense to me at least! Just do the piston lift test and you will be able to confirm the theory.

Yes, but it is might be the other way around. A single non-firing cylinder from a group of three will take the AFR up to ~18+, so it is not easy to nail down which way around it is going to be. Since you don’t report a misfire or lumpy idle, everything is probably working, but there may be a big mismatch between individual cylinders for all we know.

kind regards