Another Carb noise question

[A98Coupe]

Hope someone out there can help?

My 427 side oiler has 2 x Holley 4160 carbs and it has recently developed a high pitched whistle sound only at idle and only when the engine is warmed up. I use a homemade engine scope (piece of hose pipe) to locate the source of the noise and found it to be coming from the front carb, left hand secondary venturi. I proved this by hold a piece of clean cloth over the venturi when the engine was running and the whistle sound stopped.
I replaced the base plate gaskets for the correct 4 hole type, and that seemed to solve the problem, until the next time I drove the car the whistle came back!
I have read about the idle and main air bleeds and wonder if they could be the source of the problem.
The engine idles and runs extremely well, no hesitation, crisp throttle response, just this annoying whistle sound at idle. Has anyone else experienced this problem?
Many thanks
Stuart

[patrickt]

Originally Posted by A98Coupe

I have read about the idle and main air bleeds and wonder if they could be the source of the problem.

I don't think so. Every carb whistle I've ever had has been a leak around the base. While the whistle is whistling, if you take a rag that is soapy and sopping wet and press it hard against the base of the whistling carb, and move the rag around so you press it against the entire perimeter, can you get the whistle to change its tone, even just a little?

[Bobcat]

Stuart .... air bleeds shouldn`t make any noise . As Patrickt suggested , check for air leaks around the base . You might try some WD 40 as it will cause the engine to speed up if you spray where the air leak is . However , if the engine idles and runs extremely well as you indicated , you probably don`t have an air/vacuum leak . Dumb question ... you said you replaced the base gaskets for a 4 hole gasket .Is your intake also a four hole unit ? If so , then the 4 hole gasket will work , if it is an open plenum , then a four hole gasket can cause some problems as it isn`t supported in the center .
Sometimes one mixture screw can be off and that will cause that venturi to make a noise ... mine did . Try adjusting the idle mixture screw on that side and see if the whistle goes away ( make a note of where you started so you can have a baseline ) . I always make a note of where each mixture screw is on my carbs after they are adjusted ( how many turns out from closed ) .... gives me a baseline to go back to if I start changing things and get absent minded .

Bob

[patrickt]

... and move the soaky, soapy rag up against anything on the outside of the carb too. Press it against the idle mixture screws, the little rods that come out of the sides of all carbs, any screw that you see, bolts, etc. on the sides of the carb. Basically anything on the outside of the carb that you can squish the wet rag against do it. That should tell you if the problem is on the outside or not -- it could be a really small hairline crack that you can't even see.

[Rick Parker]

You may have a worn throttle shaft bore in the baseplate. After it begins, wiggle the throttle shaft to see if it changes pitch or is eliminated, maybe a couple of drops of 30 wt oil on the shaft. Many times the return springs are incorrectly attached at the bottom of the linkage causing the shaft to bare against the rearmost surface of the bored hole in the base plate. The spring is best attached so it attaches and pulls to the front of the engine (conventionally mounted 4bbl). Mounted at the same point as the throttle cable. If this is a new carb it is probably something else. But it is heat realated as you described, it is caused by expansion or warpage of a carburetor component. Also squirt a little carb cleaner in the air bleeds while it is running.

[A98Coupe]

Thanks for the suggestions guys. I'll try the wet/soapy rag test around the baseplate as Patrickt suggests and see if that points to the source. However, with my homemade engine scope, I couldn't detect any noise around the base plates of either carb. It was definitely coming from the front left hand secondary venturi - which is a mystery to me.

Bobcat - Thanks for the suggestion about the mixture screw - I'll try that but given that the noise is coming from the secondary venturi, the mixture screw only affects the primary fuel/air mixture right? Also, yes I do have a dual plane inlet manifold with 4 holes for each carb. This is why I changed the baseplate gaskets, thinking that this would cure the noise issue.

Rick - The carbs have only done 2000miles, so worn shaft bores is unlikely. Also, the throttle return spring is attched to the throttle linkage and not the carbs. I have tried wiggling the throttle shafts, but it makes no difference. I will try either WD40 or 30 wt oil on the shafts to see if that has any effect.

Thanks again guys - your comments are much appreciated - I will let you know what transpires.