Vacuum leaks are a potential problem area. In the 911 SC, there are SO many places that can be sources of vacuum leaks. It is a good idea to rule out many of the most common sources of leaks right at the beginning. The good news is you typically don't have to invest a lot of money to find them.
There are many potential leak sources. Here are a few of the more common ones:
1) Old vacuum lines
There are 43 quadrillion linear feet of vacuum hose in the 911 SC. Ok, not really that many, but it sure does feel like it. After 30 years and a few thousand heat cycles, the old lines can get brittle and crack. A good WYIT (While You're In There) project if you ever have the engine out of the car is to replace all the vacuum lines.
2) Cracked air box
A common story is "My car was running great. Then I started the car one day and it made a loud KER-PLOW! Now it's not running well..." Chances are your car didn't have a pop-off valve installed, you had a backfire and it cracked the airbox. (Or maybe it did have a pop-off valve and it blew anyway!) There is great debate over the necessity of the pop-off valve. I'm not going to wade into that debate here. But if this scenario describes what happened to you, check your airbox for a crack. It may be a hairline crack, but chances are it's there.
More reading:
http://forums.pelicanparts.com/porsche-911-technical-forum/689763-blown-airbox-replace-prevent-while-im-there.html Pay particular attention to Peter Zimmerman's posts #10 and #11 on CIS operation theory.
3) Problem with the pop-off valve
Another common story is that the pop-off valve is installed but the car exhibits the same poor running conditions as when there is a blown airbox. Yes, isn't it ironic that the thing that is supposed to be preventing you from blowing your airbox is actually a possible cause of the same symptoms as a blown airbox? At least the pop-off valve is cheaper than a new airbox. This can be a few different issues. First, it is possible that the epoxy seal around the pop-off valve has loosened with age and is allowing false air into the system. Another possibility is that the pop-off valve may be binding or not seating properly after a backfire. It is not altogether uncommon for pop-off valves to be installed backwards either. They should be installed with the hinge side facing the front of the airbox as you face the open engine compartment.
4) Leaks around the fuel injectors
Another common leak source is the injector sleeves and the O-rings which can harden and crack with age. Solution: replace them.
Methods for finding leaks:
The easiest method to quickly determine if you have a significant vac leak is to remove the oil filler cap when the car is running and the engine is warmed up. If the idle RPMs change when you take off the cap, this means you DO NOT have a significant vac leak. You still might have one, mind you. It's just that it might not be that big. If the idle RPMs do not change, that means you DO have a significant vac leak.
Here is a DIY way to check for vac leaks:
http://forums.pelicanparts.com/porsche-911-technical-forum/717648-finding-cis-vacuum-leaks-vacuum-cleaner-pressure-test.html
Some people advocate using propane or other combustibles to spray around potential leak sources. This is an easy method that is safer.
The final method is to take your car to a shop that has a smoke machine. The smoke has a chemical in it that is reactive under UV light and leaks can be detected this way.