Exhaust bangs

You have not give very much information on the engine.

Even so, I would speculate that you have a crack somewhere in the exhaust system.

If this should be the case,  it is urgent that you stop flying, as there is danger from CO gas and the potential for hot exhaust igniting some part of the aircraft.

Do you mean backfiring, that is unburnt fuel combusting in the exhaust? I have not had that on a 912 but in general there are a number of possible causes such as

Closing the throttle too quickly 

Mixture too lean 

Mixture too rich 

Spark too late 

Valve timing wrong 

Exhaust valves not sealing

What's the history, how old is it, has it had a recent rebuild? Has this behaviour just started? Does it run well at high power?

Are the carburettors in good condition, floats within weight, jets clean, synchronised? Any sign of the float chambers overflowing?

Carburettors are the first suspicion for many problems!

Thanks! The engine has gone close to 400 hours and I think it´s a backfireing. The carburators and the ignition sytem looks good. The engine is running very well at every rpm so perhaps it can be a valve. I saw when I checked the spark plugs that on one cylinder the spark plugs   were more "used", some coating compare to the others. 

Thank you again for valuable tips. 

Torleif

Just remember my advice Torleiff - when an exhaust system has a crack/hole, it can often backfire when the throttle is closed - this is gas igniting in the exhaust due to the crack/hole sucking in oxygen rich air. The backfire can cause damage but this is nothing compared with what Carbon Monoxide (CO) poisoning & hot exhaust gas, igniting something in the engine compartment, can do. Carefully check all parts of the exhaust system - hairline cracks can be hard to see.

I  will check! Thank you!