uncommon noise from carburetor

Dothang67 · May 24, 2006

I just finished rebuilding my carb, but after installing it in the car a strange noise came up. It sounds like if something is blowing (or sucking) air. I thought the noise was loud because the car was running without the air cleaner assy, but after putting it, the noise was still there.
So I take the car for a ride, and when I start gearing 2nd or 3rd gear it kinda shut ups, but then it suddenly appears again.

Could it be something like a valve or needle I didnt adjust correctly?
Can someone help me out?

I just checked, and it doesnt appear to have any leaks or fumes around it.

Thanks in advance

Max Power · May 24, 2006

Sounds like a vacuum leak of some sort. Did you get all the vacuum connections secure? By removing and re-installing some of the vacuum lines, any chance you cracked one?

Dothang67 · May 24, 2006

I check the 2 vaccum lines, and they arent crack, they look in good shape.

But as I check the lines I saw a small leak of water in my radiator, is there a chance, the sound could come from there?

I attach some photos (I hope this works):

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Thanks for your help

pabear89 · May 24, 2006

...From your pic's did you forget to install the choke tube or plug the hole?
A small amount of vacume is drawn in thru the choke housing to warm the coil.

PB

Dothang67 · May 24, 2006

Thanks, I will try that tomorrow morning. Maybe while I was cleaning it I removed the thing that covered it without noticing.

Sorry for my ignorance, but you mean the one in the second photo right? (top right corner) If so, with what should I plug it?

thanks pabear

Dothang67 · May 26, 2006

Does anyone knows which one he is referring to?

dennis112 · May 26, 2006

On the first picture, you have 2 possible things that you can plug, assuming you don't have them hooked up by now. The first is on the back of the carb-top center photo. It will allow dirty air to enter the carb at high speed (with the air cleaner on.) Use it or else plug it.

The second is on the rear of the choke--the threaded port (normally goes to the exhaust to bring heat up to the choke.) Carb vaccum would normally help pull heat up to the choke. Use it or else plug it.

On the second pic you are showing a carb vent that was used on pollution control cars. You can plug it too if you want, but it is not as important as the those on the first pic.

302 coupe · May 26, 2006

that choke tube hole will draw air, it might not necesarily be whats making the sound, but it is creating a substantial vacuum leak.

BTW, carbs do, to some extant, whistle. The 650dp on my car whistles quite audibly, but no vaccum leaks.

68conv4sp · May 26, 2006

Had a similar noise from my carb, with no problems or leaks as far as I could tell. My solution was to get glass packs. Can't hear the carb anymore.

Dothang67 · May 27, 2006

last questions

dennis112 said:
On the first picture, you have 2 possible things that you can plug, assuming you don't have them hooked up by now. The first is on the back of the carb-top center photo. It will allow dirty air to enter the carb at high speed (with the air cleaner on.) Use it or else plug it.

The second is on the rear of the choke--the threaded port (normally goes to the exhaust to bring heat up to the choke.) Carb vaccum would normally help pull heat up to the choke. Use it or else plug it.

On the second pic you are showing a carb vent that was used on pollution control cars. You can plug it too if you want, but it is not as important as the those on the first pic.

Thanks for the info, I really appreciated.
One last question: If I plan of using both things, where should I plug the other end?
As you explained, the second one (the one with the threaded port) needs to be plugged to the exhaust, but the other one just sits there?

Else, if I decided not to use them, with what should I plug them?

-Andrew

dennis112 · May 27, 2006

Dothang67 said:
Thanks for the info, I really appreciated.
One last question: If I plan of using both things, where should I plug the other end?
As you explained, the second one (the one with the threaded port) needs to be plugged to the exhaust, but the other one just sits there?

Else, if I decided not to use them, with what should I plug them?

-Andrew

OK, lets see if I can explain it a little better. The fitting on the back of the choke is connected by a hard line to a hole in the stock exhaust manifold (I believe specifically it is on the top of the manifold.)

This hole goes through the manifold and would connect to a second tube that leads up to the underside of the carb choke horn (seen in the #1 pic.) The purpose of both connections is to allow filtered air to warm up and enter the choke housing via vacuum. That connection takes care of 2 of your ports.

You could use a rubber plug on both of them--the one over the choke would have to be large enough to squeeze over the threads. Alternately you could use vaccuum or fuel hose and put a bolt into it to stop air draw.

D.Hearne · May 27, 2006

One other source of a carb "whistle" is worn throttle shaft bushings. Had a Holley 600 do this. Took awhile to figure where the whistle was coming from. It was sucking air in from the right side of the primary throttle shaft. Not enough to make it run rough like most vacuum leaks, but it did whistle a pretty good tune. Carb worked fine other than the noise.

Dothang67 · May 28, 2006

Thanks dennis for the explanation. After trying what you said, the noise was almost completely gone, now I can live with it

May be I could try what D.Hearne posted, but I think I am fine now.
Well, thanks again for helping me out.

-Andrew

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