Enclosed between the 2 cylinder rows, the rubber carburetor insulators get hot which can cause them to get rock hard (especially in hot climates), this can result in air leaks that cause vibrations and reduced performance.

Getting proper access to these rubber parts is a lot of work, periodical checking is therefore best done by prodding the bits you can reach with for example a screwdriver, this should enable you to determine if the rubber is still flexible.

George Georgalas kindly supplied pictures that may help a bit should you embark on replacing these rubber parts. Note that this is a big job to undertake.