What’s this spring for?

The spring is a vibration damper/support to hold the rubber mounted carburettors in place.

Can only speculate;

In your engine, the missing/unattached spring may be allowing the carburettor to sag, thereby allowing a gap between carb  & rubber flange.

Gap would allow pressurised air/fuel mix to escape causing a drop in boost pressure.

Putting the spring back restores integrity of seal and boost comes up accordingly.

I would also be checking the tightness of the flange/carb clamp.

There should be sufficient tension on that spring to prevent it from coming loose. If the hose clamp on the balance tube isn't tight enough it will rotate causing the spring to go slack and potentially become disconnected.

I would carefully check that you don't have a split in the carb mounting rubber. You will have slacken the main clamp and remove the carb to inspect it properly. It is a common failure and it will admit excess air through the split.

I don't like the standard carb mounting arrangement on the 912/914. It is better if there is an air box to support them.

Kevin Stewart wrote:

There should be sufficient tension on that spring to prevent it from coming loose. If the hose clamp on the balance tube isn't tight enough it will rotate causing the spring to go slack and potentially become disconnected.

I would carefully check that you don't have a split in the carb mounting rubber. You will have slacken the main clamp and remove the carb to inspect it properly. It is a common failure and it will admit excess air through the split.

I don't like the standard carb mounting arrangement on the 912/914. It is better if there is an air box to support them.

"I don't like the standard carb mounting arrangement on the 912/914. It is better if there is an air box to support them."

In the past I would have agreed with you, however I met a pilot & his pilot wife who survived the loss of a propeller blade in flight .

The violence of the out of balance engine prop, was so great that significant damage was done to the engine frame & firewall - in fact the engine almost left the aircraft.

The speed of the situation was such, that the very experienced pilots (both also instructors) had no time to react.

They were saved by the carburettors being shaken from the rubber sockets, shutting the engine down within milliseconds of the prop disintegration.

They were able to glide the stricken aircraft to a successful forced landing. If the engine had separated from the aircraft,  both would have been killed.

The carburettors had no air box connecting/supporting them. They attributed this (lack of) feature to the primary reason for their survival.