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Old 03-01-2006, 10:17 AM
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Q-jet

Noisy Q-jet.
I tried out different Q-jets on my new-build 360. (Don't know the spec or history on these carbs) The one who performed best at high rpm is making a vacuum-cleaner noise.. Especially at idle. What may cause this remarkable sound??

Another non-important question:
Is the exhaust sound when the secondary opens typical for the Q-jet? When cruising and then opening full throttle its quiet for a little while - then it fires like a shotgun.
Cool sound with headers and free-flow mufflers!

Q-jet hater/lovers opinion?
Thanks, Pettersen

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Old 03-01-2006, 02:07 PM
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Qjet lover here.

Qjets are remarkably accurate carbs, but are a bit complex. The shotgun exhaust is not normal. Neither is the vacuum cleaner sound

A proper rebuild along with proper jetting should fix both. Sounds like a strong vacuum leak at idle and a jetting mismatch or a leak in the secondaries when they're open.

On the driver's side there are usually ID numbers stamped on the carb. They should start with 70 or 170. Post the numbers and maybe we can identify their origins which might help us determine which one is the better candidate for a rebuild. All Qjets are very similar and all will do well when properly tuned, but some are slightly better as far as modifications are concerned.
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Old 03-01-2006, 07:05 PM
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The vacuum sound is normal if you ask me. Every single vehicle I've ridden in with a Quadrajet has the classic sound. Its not like a vacuum leak hissing sounds, its a deep intake sound. Perfectly normal. My 5.0L makes a similar sound from the throttle body when i get on it.
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Old 03-01-2006, 07:24 PM
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Oh, yeah. That noise. when he said vacuum cleaner I was thinking the whole whirring motor noise of a shop vac. But the rushing sucking air noise is quite normal. Almost a metallic hiss. Quite normal.
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Old 03-08-2006, 07:19 AM
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Thanks, good to normal..

I've tested again and I agree - the vacuumsound in this carb is normal, but much louder!
I took a closer look at the noisy carb noticed that the air horns was'nt very smooth.
Image 1
Image 2

Is it possible that these minor imperfection make this sound?
Does different jet-sizes make different sounds?


Pettersen
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Old 03-08-2006, 02:54 PM
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No, the sound is there on every single carb. Should be a very loud sucking noise that sounds almost like an air gun on an air compressor. That is the fact that the engine is drawing a huge vacuum at idle and the incoming air is being violently rushed past a tiny sliver of space beside the throttles. Its normal. The velocity of the air at idle past those little boosters isn't what's causing it.

While you're under the hood with the engine idling, punch the throttle open with your hand. You'll notice for a split second that rush of air noise goes away but then returns when you let go of the throttle.

Then go do the same with 10 other cars. Take the air cleaner off and listen to it idle. It sucks hard. Even on EFI...same thing. Its just air rushing past the throttles because of the intense vacuum in the intake. What may make it louder on this car is that it my provide more vacuum at idle, or your throttle blades are closer to being closed. On a race car I doubt you'd hear much at all. They make very little vacuum at idle and the throttles have to be open a lot just for it to idle. The increased space for flow and the fact that there is little vacuum at all make it a quieter sucking. Every engine will be different, but it wouldn't matter how many Qjets you put on your engine, if its tuned the same way, it will make the same noise. If you find a quieter engine and put your qjet on it and tune it to that engine, the noise will be quieter since you've tuned the carb to that engine's vacuum profile.
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