hey guys i need some info....i got a holley 750VS and acouple of time soon after i shut off the engine i would hear this hissing sound that sounds like it's comming from the carb. everytime i hear it i got the hood up. the sound lasts about 3 sec and i am almost positive its from the carb, if not its gotta be something beside it. it sort of sounds like its letting out air or something, anyway can u guys tell me what the heck is doing this and why? and is it normal??
thanks

 

Sounds like a vacuum leak. check your carb base plate and manifold base to make sure they are flat and not warped from over tightening(carb). Check any vacuum lines going into the manifold or carb and snug down the intake manifold to make sure its sealed around the ports. Might also want to try and spray some WD 40 around the carb and intake ports while engine is running if idle goes up you ahave located the leak. Hope this helps.

 

If it's hissing after it's shut off, it shouldn't be the carb since it doesn't store vacuum.
How about the brake booster/check valve or the a/c ( if you've been running it).
Air conditioners will hiss after shut down until the pressure equalizes.