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Matt Harwood

Singing carburetor

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So I just returned from Hershey and put nearly 1000 miles on my 1941 Cadillac, and it delivered a near-flawless performance. It never missed a beat, but has developed a very odd high-pitched scream, almost like a siren. It happens after some time on the highway or at elevated speeds (above 40 MPH for more than 3-4 minutes) and is directly connected to the accelerator. Larger throttle openings and it diminishes and goes away, but with closed throttle it's a very high-pitched squeal, almost like a police siren. I can vary the pitch by changing the throttle opening. It is most noticeable as I am coasting down an exit ramp on the highway with the throttle completely closed. By the time I come to a stop at the bottom, it has usually faded away and doesn't come back until I'm back at cruising speed. Since it seems to bleed off, I can't stop the car, open the hood, and find the source quickly enough, but it is quite annoying. I believe it is vacuum-related and it's most definitely in the carburetor since it's tied directly to my right foot--I can almost play a tune with it by twitching my toes.

Any thoughts? It's very weird, but perhaps there's a bleed valve or something in the carburetor that's not working correctly to relieve vacuum pressure at cruising speeds?

Thanks!

PS: If it's any help, the carburetor is a Stromberg AAV.

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Sounds like an air leak somewhere? Would be strange that air may be able to be sucked in, causing the noise, but fuel does not leak out? Strange, but not out of the realm of probability.

I would check the tightness of all screws in the carburetor air horn.

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I've noticed that some carburetors can tend to make noise from the air passing through the throttle bores. Many don't, but some do. Not sure why, even for the same model of carburetor.

Haven't heard any as loud as you mention, though.

What KIND of gasoline are you feedin that Caddy???

Just some thougths,

NTX5467

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I would also back a vacuum leak somewhere, maybe even an inlet manifold leak. Spray a little water around the carb and inlet manifold while it's running, or "Easy Start" (sparingly), if you're not faint hearted. Listen for a change in engine note.

Al

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Record it!!! ;)

Sounds like you have the makings of a rock group. Now if you can just get a squealing bearing, maybe a clucking valve, and a slapping piston to go along with the singing carburetor; your fortune is made. The "Singing Stromberg and the Autos" group. You need to teach it a few tunes, and then cut a CD.

All joking aside, it is probably a vacuum leak somewhere. Since the vacuum will change with the position of the throttle plates in the carburetor, the sound will also change. I would first check all vacuum lines, then the gaskets around the carburetor base and the intake manifold. Since it doesn't start right away, it may be heat related (expansion); so maybe let the engine idle to normal operating temperature and then check for leaks. It is also possible, although rarer than an Ebay auction without the word "rare" in the title, for carburetor icing to cause a whistle. If this is the issue, slowing down will allow the underhood heat to melt the ice and thus kill the whistle. Carburetor icing can occur, even in temperatures up to the 60's if the humidity is extremely high and the carburetor is running too rich. An electric fuel pump causing too high a float level can cause this issue.

Anyway, a couple of things to check. Send me an autographed copy of the CD!;)

Jon.

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Sounds to me like a fan belt squeal or a bearing about to burn out. Such as a generator, water pump or fan bearing. I don't know why the noise comes and goes but they do act funny sometimes.

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I've had that happen before to me. The flange nuts loosened up slightly allowing air to leak around the base of the carburetor. Thats where I would start. See if you can get it to act up while its not moving and go from there. Good luck!

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You have an automatic choke...correct? Just for fun, follow the hot air tube from the automatic choke housing, down to the choke stove on the engine (wherever it is). Wrap a towel or cloth of some sort, around the junction of the hot air tube and the choke stove, start the engine, and speed it up to see if you still hear the squeal. I had a very high pitched squeal in my engine and only after a couple years did I realize it was caused by the vacuum within the hot air tube that pulls the heated air up to the carburetor. The connection between tube and choke stove wasn't tight, and that's where the whistling was coming from. At least, if this doesn't cure the problem, you'll be able to cross it off your list as a possibility!

Edited by Jon37 (see edit history)

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I just wanted to follow-up and let you all know the problem has apparently been solved. We pulled the exhaust manifolds to replace a gasket, and since we were in there, we pulled the carburetor and sealed all the fittings, checked all the hoses under the dash, and pretty much traced every vacuum source. After a test drive, the siren sound was still there. A friend suggested that maybe it was a U-joint, so we pulled those and sure enough the front U-joint's needle bearings were pulverized, so we put in a new U-joint. Part of the sound went away (the part that sounded like a chattering fan belt), but the siren remained. In desperation, I started hosing down the upper end of the engine with brake cleaner and found that the phenolic spacer under the carb was somewhat porous. We put a light coating of gasket shellac on it to seal it up, and it seems to have cured the problem.

So you guys were right, it was definitely vacuum-related. Just who would have thought it would be seeping through like that?

Thanks for all the advice, particularly the advice about making sure the carb bolts were tight. That's where we started and what led us to the actual culprit. Nice work!

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The follow-up, with solution, is appreciated. You would be amazed how many folk ask, but never report back with the solution. Yes, I would never have thought of a porus spacer. Seems to me that spacers should not be porus?

The manufacturer should be notified of the problem with its product!

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