Vacuum Leak at the distributor

[sk8er32283]

I was under the truck messing with the throttle position return springs and heard a vacuum leak that sounded like it was coming from the distributor. The engine is from a 1978 truck and has a "regular" distributor with vacuum advance timing. If I had to describe the sound it would be a hissing sound that got louder as the engine rpm's rise. I am mostly a diesel guy so forgive me if my lingo about distributors is off a bit. Please correct me if I am not calling something by its normal name. Here is a picture of the distributor. I have always felt that the truck was not performing up to snuff and maybe this has a little to do with that issue. Or it could be that the engine is from a 78 when they had all the emissions stuff designed into the engine. Please let me know if there is any other info you need from me to help out with the trouble shooting.