Tuning my engine, after I shut it off, I hear a hissing sound. I initially thought it was the radiator, but, upon further examination I discover the sound is coming from the carb.

Could this possibly be fuel boiling? If so, is that a bad thing? The carb is a Quadrajet with a standard gasket and the standard stainless heat shield. Should I be concerned about this?

Any problems with starting the engine?

No, the engine seems to start ok, not always on the first turn of the key, but, it does start.

If you're happy with the way it starts--ignore the hissing.

If you're happy with the way it starts--ignore the hissing.

Schurkey,

I would prefer the engine to start immediately. On occasion it does hasitate when starting.

my brother ignored the hissing one day and the snake bit him.

Do you have a vacuum canister (the ball kind on the fire wall)? if so it could be a vacuum "leak" or air leaking back into the vacuum canister. It also could be vacuum leaking past the master cylinder if you have one. How long does it hiss?

No "ball" canister. It is definitely the carb, no doubt. It hisses when it's hot. I haven't listened to it until it stops, but, when it cools down, the hissing stops. By the way, I do know how to spell hesitates, LOL, the hasitates was a typo!

Are you by any chance running a stock cast iron q-jet intake for 69 (cast id 948) & 69 (cast id 067) 396 that has an exhaust crossover that runs under the front of the q-jet that pre heats the carb?

If yes do you have the steel shim inbetween the gasket & carb or inbetween the gasket & intake?

The steel shim goe's inbetween the carb & gasket and if its inbetween the gasket & intake manifold maybe its leaking vacuum if it's making noise when the motor is running.

Or even if the metal shim is installed correctly inbetween carb & gasket there can still be a vacuum leak between the carb & steel plate when the motor is running if the base of the carb is warped.

If the gurgerling/bubbling/boling noise is present with motor up to operating temp but not running then maybe it's on the edge of or actually running too hot & boiling the fuel.

Maybe the bowl vent mounted on the top/front of the carb is adjusted too tight making noise from heated fuel vapors escaping past it.? If you find its that then re-adjst it to factory spec and that may stop the noise. The bowl vent is supposed to be cracked open at idle which is activated by a small wire rod off the accelorator pump linkage.

Do you have a mechanical temp gauge?

If yes what temp is the motor running,esp when you 1st shut it down & it heat soaks for a few mins which is when boiling of the fuel in the carb would be most likely to happen ?

Maybe it's the power brake booster valve thats gone bad and your simply hearing the booster loosing vacuum slowly through the hose that hooks to the intake manifold or back of carb which is where the hissing is your hearing.

Scott

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Scott,

Yes, I have the iron intake with the exhaust crossover. The steel plate is between the gasket and carb. Yes, the noise is present when the engine is hot and not running. I do have a mechanical temp gauge under the hood and it's around 190-200 when I shut it off and after a few minutes it'll hit 220. I'll check the bowl vent on top of the carb. By the way, the booster is new, I don't think it's leaking.

When i mentioned the brake booster leaking vacuum after the motor was shut down i was reffering to the check valve in the booster vac hose poss being defective allowing the booster to bleed off vacuum through the intake which could then be heard through the intake/carb.

scott