Loud air moving sound within carb with cleaner removed

[Dave Fehrle]

While checking my Carter carb with engine running, I took the air cleaner off and there was a very loud hissing sound of air. I placed a piece of paper over the carb and it didn’t move up or down, is this normal?

[Dennis P. DiBari]

Hi Dave,

     It sounds like you checked everything.  As Jason said, there is a normal vacuum sound when you remove the air cleaner.  I have a 4-barrel Rochester on my 64, so it is a bit different than your Carter.  If you checked all the lines that Jason mentioned, then you should be OK.  The booster, vacuum advance, PCV valve are all big vacuum items.

Dennis DiBari, Cottekill, NY

[Jason Edge]

With the air cleaner off you will get an air sucking sound. I listened to your video not sure I hear anything too out of the ordinary. With cleaner on do you still hear loud or pronounced air sound? 
If all of your vacuum hoses are good and tight and you have checked the devices they operate (e.g. vacuum advance, brake booster, throttle check valve (if equipped), AC idle speed-up control, if equipped, etc. If you have issues inside the cabin I don't think you would hear from engine compartment. For that with car running stick your head under dash, with AC/Heat on moving controls then try to set unset park brake via gear shift (obviously not with head still under dash). 
You should be able to isolate a specific leak, but again you do get a sucking sound when you take that air cleaner off. I will try to give mine a listen with filter off and compare but it is raining cats and dogs here right now and cannot get car outside.  

[Kurt]

Is the car running badly? Sounds normal to me.

[Dave Fehrle]

Had some time today to check vacuum hoses out of the manifold as well as the one into the carb from the dist check valve. All 3 were good with no hissing. Even took them off and pushed air thru with no holes or splits. I took a video of the carb while the car was running to get the sound from the carb(see attached). 
I believe my heat riser is stuck in closed position, thus causing left exhaust to flow thru the manifold and that is what is making the sound. Any other ideas?

IMG_5205.MOV

[Dave Fehrle]

Thank you for all the helpful ideas, I will start the search and report back!

what size is the small vacuum line coming off the manifold?

[Mike Travers]

Not sure about the paper over the carb but the hissing is not normal and is, almost always, a vacuum leak. In order to help find the leak, you can go to your parts store and buy about 3' of rubber vacuum hose (any size) and use it as a makeshift stethoscope. Once back in your garage, put one end in your ear and move the other end across your running engine and the hiss will get very loud when you move the open end of your hose over the leak. Viola! You've found the leak. 

I hope this helps you find your issue.

[Dennis P. DiBari]

Hi Dave,

     As Chris said, put your ear close to each of the vacuum hoses around the carburetor and intake manifold.  With the heat, they become brittle.  Look specifically at the 11/32 vacuum hose to the check valve off the power brake booster (it must be 11/32 vacuum hose, not 3/8 heater hose!!!).  Also, look at the vacuum hose cluster coming out of the firewall - they go to the heat control valve and the vacuum pot near the heater core.  My hoses cracked at the firewall, and I had to repair them to get my heater to work again.  Good luck and let me know!

Dennis DiBari, Cottekill, NY

[Chris McBride]

Check for vacuum leaks, a common source for hissing noises. The vacuum hoses commonly become hard and brittle after enough time in the heat of the engine compartment. They often harden and leak at the end points. You can sometimes snip of the offending end and reattach the more pliable remaining hose. Vacuum hose is also comparatively cheap.

I hope that this resolves the hissing noise since the other possibility, a leaking intake manifold, is a far more problematic issue.

I have no idea if paper should remain immobile above an idling engine.

I hope that you find a leaking hose since another possible issue, a leaking intake manifold, is a more problematic issue.

Let us know what you find out.

HTH

Chris

Chris