Engine vibration on ignition circuit A

How have you checked the ignition? You may have a bad connection to one spark plug, there might be a problem with one of the resistive plug caps.

Assuming that it is fuel related, I would check for blockages of any of the jets including the small drillings in the venturi just down wind of the butterfly valve. If you have poor atomisation of the fuel you will not get an even distribution of air/fuel to both cylinders and the effects of this may be made worse on a single ignition circuit.

There is no problem with ignition, I've tested everything ... Vibration is not caused by single spark because on B circuit it's ok no vibration ...

What is the mag drop numbers? Mainly the difference between the two circuits.

Thanks for your response, there is no big difference between the 2 circuits, it's only vibrations ...

Are you saying that you have swapped the ignition modules and the vibration does not move with the module? If so, have you checked the condition and the gaps on the flywheel pick-ups? If the fault remains on the A-side could it be the supply to the ingnition module? I know that there have been reports of breaks in the red wire close to the modules.