Our classic cars were of straight forward constructions as well as the electrical components. Nowadays many of us add electronic components like LED, gauges, ignition etc to the existing circuitry.
I guess adding high impedence of different origin can cause interference to them, and as a result bad functioning.
Has anyone experienced this and if so is there information on how to overcome ?

I have.
But give a hint as to the issue!

Electronic ignition : engine starts but when further opening the throttle, no extra revs, engine starts to sputter and excessive petrol is ignited in the exhaust.
I think that the drivers of the LED 's I used in the headlamps are not protected against the peak currents put on the earth connection by the ignition circuitry.

Disconnect the headlamps and see if the problems exists.

Have got led arrays for all lamps apart from headlights, no issues.

Some of these

Check all of your grounds.

An idea, do the led drivers create a magnetic field?
That may interfere with the hall sensor in the ignition.

Have a magnetic clutch (from Air con pump) on the engine fan, when first fitted and used car cut out completely, resolved by us using Stainless steel for all fan mounts, now works ok

I now have only an estimate of the problem, sometime next week I will open different LED circuits to find out which one causes the interference. The ignition fires the coil on plugs via the minus connection.

Quick google

http://www.ledbenchmark.com/faq/LED-int ... ssues.html

If its RFI, MUMetal case for the drivers, but he RFI may also radiate thru the wiring?

In the 12 V applications its not the interference of a switchmode power supply, but interference through the wiring by the LED driver. Will find out that later this week.

Use these https://www.ebay.co.uk/itm/20-Pieces-Cl ... SwnoVa05zo