Can't find source of Whistling

[bd]

If you selectively spray directly into the airhorn, does it change?

[LTZ C20]

Haha yea!

[bd]

Not just spraying into the horn in general, but directing the nozzle at specific points (bleeds, etc) with short bursts.  Try to determine if the whistle is coming from "inside" or "outside" the carb.  If so, any chance the air horn gasket is the wrong one or torn (or missing)?  W/o reinventing the wheel, try spraying directly around the pintle shaft of the EGR. 

[LTZ C20]

I have sprayed the EGR and nothing happens. I recently did a rebuild on the carb so I know the gasket is there. I remember putting it all together. I don't know where inside the carb its coming from, I just know its from inside the carb.

[bd]

With the engine off, using a good flashlight, look down the throttle bores at the throttle plates.  Focus on the "roundness" of the plates and whether they seat in the bores all the way around, especially where they cross the throttle shaft - look for good throttle plate-to-bore contact all the way around their circumference.  Are there any gaps?  With the engine still off, grab the driver side throttle linkage and open the throttle a few degrees.  Wiggle the shaft fore and aft.  How much play is there?  Can you see the plates wiggling fore and aft in the bores?  How much?  Did you have any trouble setting the engine idle or with the throttle plates sticking slightly at idle?  Do your best to answer all 5 questions.

[LTZ C20]

hahaha, is this for a grade? i will do this as soon as i finish eating dinner. bout 30 min.

[bd]

You already got your 100%!  This would be EXTRA CREDIT!!!   

Nah!  Just trying to figure how much actual wear is in the throttle shaft and at the plate margins.  Trying to get my arms around the melody your carb is playing.  I'm a bit more familiar with QJets than the Edelbrock AVS.  Tryin to diagnose over the phone can be a challenge  .  What I'm really after is to try to isolate the noise to a specific part of the carb.  I've had QJets whistle (rarely) from the air bleeds, past the plates, and through the idle passages for various reasons (ie, warped air horns, wrong gaskets, excess shaft wear affecting plate indexing).  But, not often, suggesting that circumstances have to be just perfect for it to occur.  Sometimes just changing the throttle closing a little bit solves it (as long as the gaskets aren't the issue).

Of course, I guess you could always get earplugs!    But, it would be a lot more fun to solve it....    Just want to see you get the product you're after... after dinner. 

[LTZ C20]

Haha well my carb is a Edelbrock Performer. Sorry if you didn't know that. As for the extra credit: 1, the gap around the plats appears to be equal. Correct me if I'm wrong but if there was no gap then there would be no air fuel mix and truck wouldnt run. with a small gap, air and fuel always pass through while its runnin, creating idle? I think right? 2, the linkage moves fluidly and i can see the plates working together. 3, when we rebuilt it, we had trouple because the plates were sticking so after carefull re-adjucment we got the plates alighned so that everything moves smoothly and nothin sticks. my unlce, who knows so much more than i do, thinks that something is worn so air is slipping past and whistling. not sure tho. how does any of that help?

[bd]

Kevin,
The Readers Digest version: On a carburetor, at idle, a metered amount of air flows past the throttle plate (adjusted by the idle speed screw) and air/fuel emulsion flows into the air stream through the idle ports just below the throttle plate.  Generally, no fuel flows past the throttle plate at idle.  Off of idle, transition slots or holes above the throttle plate are gradually exposed to manifold vacuum as the throttle opens to provide additional fuel/air that mixes with the main air stream until sufficient vacuum tips the main nozzles to start flowing.  There are various calibrated air bleeds that work in conjunction with the fuel metering to maintain proper mixing and fuel delivery for any given manifold depression.  If the right combination of air pressure/velocity/expansion/resonance occurs, it can do what yours is doing.  I think your uncle is absolutely correct!  The cause of your problem is wear.

As to where my questions were leading, look at the attached sketch.  The new plate is on the left; the worn plate on the right.  Gaps caused by wear can alter the air flow.  If you get the right combination of "gap" it whistles.  The wear you found and compensated in the shaft may well be the cause.  In the long run you may have to bush the base.  And, if your throttle plates look anything like the drawing at right, the plates will need to be replaced too.

[bake74]

     Just a thought, the intake you put on that was new, it was not a higher than stock one was it ?  A higher one might act like a throttle body/carb spacer (vortex effect) and make it whistle.
     If you could it would be great to take a video of it so we can hear the whistle you are talking about and post it.

[LTZ C20]

Well I have so new news. I FIXED IT! I bought a new set of carb stud bolts and installed them. I also found a blow gasket with a worped cover plate. So straightened the plate, sanded it, and reinstalled it to cover the unused port. <---This was causing a vacum leak. With the vaccum leaked fixed and new stud bolts, I thought it would create a different pressure when bolting the carb down. I found that sitting flat on a surfaces, my carb my be slightly worped, only off by thousands of an inch. So with a good base gasket, a fixed leak, new bolts, NOTHING LEAKES! The whistling wasn't there when I started the engine. After some vacum tuning with a vacum gauge, adjusting my mixture and idle, the engine now runs great. Haven't had any problems. I did this yesterday afternoon and since then all is well. I have smooth acceleration, no hesitation, no abnormal noises, idling is great. I may be buying a new Holley carb if my unlce wants to. We are going to see how it continues to run. I'm very pleased with the results. A new carb would be nice but, as of now, this Edelbrock is working very good. I have decided to stop the engine work and continue witht the rest of the truck. It purrs like a MEAN kitten and souns great from the pipes, got a good rumble. Thanks all for your help! She's got quite alot of go go in her and she moves when you want her to! 

[bd]

That's GREAT news!!!   

[bake74]

      Vacuum leaks are like trying to find a dead short somewhere in your vehicle.  It takes forever to isolate and fix, but in the end you feel like you just triumphed over raging tiger in a roman gladiator arena.   
      Or your so frustrated that it took you so long you just feel like sticking a piece of dynamite in the gas tank, lighting it and walking away.  Guess it depends on your personality.

[Irish_Alley]

heck i would of just cut the mufflers off so it drowns out the noise

[LTZ C20]

I'm bout ready to chop my catalytic converter. smog.... -__-