Fuel Mixture Tuning Tools

Motivated by Pekka’s idea of putting his car on a rolling road for dyno testing and carb tuning, what are tools people have used for tuning the air/fuel ratio on their cars?

It would be wonderful to see some rolling road graph results and measurements from other methods.

Innovate Motorsports has AFR kits with different mounting options for the wideband sensors. I use it on my Mark V with a tailpipe clamp and put the gauge on the passenger seat. This allows air/fuel ratio measurements under all driving conditions (not just at idle or in the garage with no load at the wheels). Other people put one or more bungs in the headers.


Gunson Colortune is a fun qualitative aid which I have used in the garage with car parked. The results have been consistent with expectations but I prefer the Innovate Motorsports quantitative results and the application on the road.

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Uni Sync flow meter has been reliable for decades balancing pairs of carbs and following the SU tuning guidelines which rely on ears and eyes. Decent qualitative approach in my experience. Preferable to the Gunson Colortune for my approach to tuning.

Skinners Union Tuning Tool Set is helpful, particularly when having the SU Workshop Manual for Carburetters and Electric Fuel Pumps.

I used this CO meter many years ago but I will admit that the job is easier in the MkIV and SS Jaguars because the twin exhaust system gives you independent measurement of the front three and back three cylinders. A point I made in this rather old YouTube.

Mortortest Exhaust Gas Analyzer - Page 1 - Home Mechanics - PistonHeads UK


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I have a set of four. :smiley:

I did use them on the Ser 1 E-type and later Ser 3 V12 E-type (four carbs, four exhaust manifolds) and I find them good for debugging, but with modern fuel not so good for adjusting mixture, the flame turns yellow only when opening the throttle or it’s already way too rich.


Have that as well. :slight_smile:

It’s s good tool but I also like the hose on my ear, it’s actually very accurate and does not restrict the air flow.


Please allow me to visit from over on the XK list.
Here is a picture of a home made screwdriver type tool for adjusting the fuel mixture screw on a SU H6 type carburetor.

The mixture screws are on the very bottoms of the carburetors, and cannot be easily seen in the narrow XK engine compartment, and while trying to adjust the screws on a running engine my normal screwdriver kept slipping off.

So I ended up making this little screwdriver having a cylindrical part which surrounds the screw head, allowing easy one-handed mixture adjustment without worrying about trying to hold a screwdriver upside down under each carburetor, without it slipping off the adjustment screw.

The tool was made by cutting and filing a flat piece of steel to shape, with a short piece of copper tubing soldered onto the screwdriver end.
The hex shaped portion can be used to first remove the protective cap, exposing the mixture screw.


A carburettor specialist I once knew used a modified stethoscope. You just remove the part which the doctor places on the patient (always cold!) and insert a short metal tube (4" to 6") to provide rigidity. Once it’s in both ears all extraneous noise is cancelled and you only hear the carburettors. They can be bought quite cheaply.

Love Gary Grants tuning tool.
Stick your finger up your exhaust, (yes, yes, I know), and if it comes out black then read on. If it comes out clean then discard this post. I have been living with SU carbs for more than 50 years and with the old leaded fuel, the traditional method of raising the piston a fraction to see if it speeded up or slowed down worked well. In fact a light grey exhaust colour verified that all was well. I have found that this method using the complex mixture of hydrocarbons of modern fuels does not respond satisfactorily resulting in sotty exhausts, excessive cylinder carbon build up and black engine oil. At a recent removal of the head on my 3.5lt Mk 4 I was shocked at the amount of carbon build-up, resulting in a top of piston layer about 1/16” thick with curled up edges ready to break off and get under the valves. The oil also used to get very black.
A friend of mine who is an ex RAA technician showed me a method which I have found to be far superior for tuning to Stoimetric. (ideal fuel/air ratio) – so I have called it the Barry Obitz method.
Here he hangs a piece of paper from the back bumper to hang over the end of the exhaust pipe using a clothes peg. The carb is then adjusted very gradually till the cracking sound of the paper under misfire is reduced to a minimum when the engine will be running as close to possible to stoimetric. It is very easy to do and I have found to be very effective. The oil remains clean and as can be shown from the photos the plugs are not sooted and my 2 fingers from each exhaust after a 100 km combination urban/country run show almost modern car exhaust cleanliness. In fact, the pictures show that a further slight lean-ness of the mixture could well be beneficial - will get onto that right away.


I like it!
I too listen for engine misfire on each exhaust pipe and your paper additions are an interesting improvement.