[Bobboloo]

I think what usually happens is that the idle and fuel mixture gets clogged at the end of their tubes by carbon and unburnt fuel making it hard to adjust them. The only rubber part that could deteriorate would be the accelerator diaphragm. The only other rubber parts are washers.

Another possibility is the throttles. They can get sticky or wobbly (if they have excessive miles)

Rebuilding is easy. But you might try cleaning the throttles with a little B-12 Chemtool. Also you can use the B-12 to clean the tubes where the idle and fuel mixture screws screw in.

Bobby