why do vaccum leaks in the rats nest mess up idling?

 

I believe the only place that air should be going into the engine is thru the carb, anywhere else causes stumbles and misses.

 

not sure how it applies to a carb, but in a fuel injected engine, vacuum leaks typically means the engine is drawing in unmeetered air, so the computer thinks there is less air in the engine then there really is, offsetting the airfuel ratio, and making the thing not run, or run lean as hell.

As for carb, it probably offsets the air/fuel ration as well.

 

Yeah, most of the vacuum lines are there to keep pressures dependent upon the pressures already in the engine. If there's a leak, it introduces faulty readings and the fuel mixture (and even ignition) can be effected accordingly.