If you think it's lean, try pulling the choke out briefly while it's misfiring. If the situation improves then you have air leak/need card rebuild or adjustment.

If it doesn't make a difference, maybe try a compression check as a badly sealing valve can make misfiring sounds.

Guys the problem is solved

The gap of the point was faulty, and it caused damage on the touching surface
I have changed the leads, spark plugs, points, condenser
Did valve timing

Now it runs pretty smooth... I rev the engine till redline and no funny sound occurs...
The landy deserves care and responds