Hissing sound at idle when hot? Why?

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1966DartConvertible

1966DartConvertible

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During the run yesterday, I notice a distinct hissing sound seem to be coming from the carb/manifold area. This only happens once the engine is fully warmed-up.

I cant pin point exactly where, just wondering what are the possibilities? Vacuum hoses are all new. Carb gasket and o-rings are new. Air cleaner is tight and does not wobble/move.

The engine drives fine, but idles a bit erratic once the hissing sound started. The hiss goes away as soon as I hit the gas...

Any idea where i should start?

Thanks.

Syed
 
sounds like a vacuum leak. Crack in the manifold or carb base one it gets hot opens? Should be able to pinpoint it with a spray can of starting fluid carefully sprayed in that area
 
Please use water in a spray bottle to try to find the leak. Generally, it's not a good idea to pour gas on a fire.
 
Knowing what in the heck you're working on would help. Some models have vacuum switches that open/close with temp changes.
 
A trick I showed my Brother to find a vac leak was to take a piece of hose (like garden hose) about 3 feet long.
Put one end up to your ear and sniff all around the intake with the other end.
You'll find it real quick.
 
Redfish:
Knowing what in the heck you're working on would help.
Click to expand...


AKA “66 Dart Convertible” and photo of his freshly restored car are two clues, third clue being question posted in /6 section of website. Look here.

Syed, I also think you have a vacuum leak related to thermal expansion were hiss diminishes with throttle opening, increased rpm, and a choppy idle.

Hose to ear method may work if air stream around intake from fan is not too great, where you are listening for a hiss. Spraying water may be your best bet, starting fluid second as these methods won’t damage your freshly painted engine parts, as possibly carburetor cleaner will.

One more place to listen to would be that remote oil filter and its related plumbing.
 
wjajr said:
Syed, I also think you have a vacuum leak related to thermal expansion were hiss diminishes with throttle opening, increased rpm, and a choppy idle.

Hose to ear method may work if air stream around intake from fan is not too great, where you are listening for a hiss. Spraying water may be your best bet, starting fluid second as these methods won’t damage your freshly painted engine parts, as possibly carburetor cleaner will.

One more place to listen to would be that remote oil filter and its related plumbing.
Click to expand...

Also, dont know if this might be related. I cannot get the idle to go below 750rpm even with both the curb idle and fast idle screw not touching the cam...

Syed
 
Also, dont know if this might be related. I cannot get the idle to go below 750rpm even with both the curb idle and fast idle screw not touching the cam...
Click to expand...
Yup, she is sucking air somewhere.
Vacuum leaks will cause high idle that can’t be lowered until it is found, and fixed.

Happy hunting.
 
Syed,

Dan has a few good ideas listed on your post over on slantsix.org as what may be happening.
 
i have a similar problem with my slant....only when warmed up idling. Never did it before i rebuilt the car now it sounds like air is being sucked into the carb throat weird because its a whining sound (imagine a crappy supercharger). Wonder what it is cuz it doesn't affect idle or drivability???
 
no i didn't it....can't figure it out for the life of me! And it hisses and moans with air cleaner on or off so weird:/
 
TrailBeast said:
A trick I showed my Brother to find a vac leak was to take a piece of hose (like garden hose) about 3 feet long.
Put one end up to your ear and sniff all around the intake with the other end.
You'll find it real quick.
Click to expand...

Found the "hiss" to be coming from the "bowl vent valve" on top of the carb float bOwl. Is this normal? Is it supposed to vent out air constantly like that when hot? It does not affect idling quality that much now, just the annoying hiss...

syed
 
Do you have a return line? Also how close is the fuel line to the exhaust? Just a guess but is it possible that the fuel is getting to hot and boiling? and the valve is releasing the air?
 
No, hissing from the bowl vent is definitely not normal. It sounds as if your carburetor may have an internal fault, or perhaps a leak at the economizer diaphragm or its cover, which is right next to the bowl vent and a much more likely source of vacuum hiss.
(Toldjya you should've let me send you that new carburetor…! :lol: )
 
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