Lean mixture

[jan.bauwmans]

Went to the mot inspection and values of exhaust are showing poor mixture.. What can I do to improve this ?

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[rmel]

Yikes! That's an AFR = 20.58 mighty lean. And if so, not a
healthy condition. If true there are a number of possible
things that could result in a high Lamda, possibly a bad
vacuum leak.

Do you have a high idle?
Is it difficult to start or runs rougher than in the past at idle or at RPM?

Check all possible places for a vacuum leak;
Brake Vacuum boost line and brake booster
Carburetors are tight to the intake manifold and Carb throttle base plate is tight to the Carb body.
Cross-over line between both Carbs is not cracked

Also, check that your exhaust header(s) didn't loosen up.

You should also aim a Infrared Thermometer at each exhaust port looking
for any unusual cylinder temperatures.

[jan.bauwmans]

Thanks for your comments.

She starts and runs good at idle and also while driving. I actually was not aware or felt the lean mixture problem until I went to MOT inspecrion.

She has rebuild carbs, petronix and valve clearances are ok. All 4 cylinders gave more or less same temp reading.

How you can easily check for vacuum leaks ?

Rgds

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[rmel]

I assume MOT stuck their probe up the exhaust pipe.
So any air leak in the exhaust system is also a possibility
for a high Lamda reading. Exhaust header, coupler from
header to heat-exchanger, lot of parts in that path that
might be worn or loose. Then the heat-exchanger itself
or muffler. Look for rusted out spots and check the bolts
and copper gaskets, and the darn coupler seals.

As far as vacuum leaks, running good and no signs of high
idle, might not be a vacuum leak. Nonetheless, but make
check that your carbs didn't loosen up from the intake manifold.
Check your brake vacuum boost is working, e.g. put your foot on
the brake with some pressure BEFORE you start, then start the
engine, as the engine builds vacuum you should feel the brake
peddle drop more as assist kicks in.

Given you don't seem to have any signs of a serious vacuum leak,
seems more likely an exhaust leak.

[ChickenPinz]

How you can easily check for vacuum leaks ?

A common way to look for vacuum leaks is to spray brake cleaner on the carbs and around their bases. You'll hear a change in RPM if the engine draws in those fumes. The brake cleaner dries without residue, BTW.

Check the vacuum hose that goes to the brake booster. Disconnect it at the engine, and cap it to see if the idle RPM changes. Same for the vacuum hose that goes to the distributor.

Another, but harder to check, possible vacuum leak location is the crossover hose between the front and rear carbs. It's under the air-shroud sheet metal, so not as friendly to check. And then there's the intake gaskets, but we're getting further down the path of unlikely causes.

And, always, look at your spark plugs. That's the classic way to judge lean-versus-rich mixtures.