Twin Carb Whistle

[bLinded_]

Hello,

 

I have this question that has been in my mind for quite a while now (since running twin DHLAs) and thought I might actually as here if anyone has information about it. Basically, when I apply light throttle there is a whistle sound that comes from the carbies. If I open the taps right up, it stops but as soon as I take my foot of (round 0-50% throttle), the whistle is constant. I had a listen to each of the throats with a bit of hose, and it seems to come from all four.

 

Now I was wondering if this is a normal sound with side drafts, or could it be something more...serious ;) ) I have heard a couple of varying stories, yet I really would just like other peoples thoughts, especially from the like of the guys who have been running side drafts for many years :( In particular, DHLA40s. I have heard that the whistle is caused by air rushing through the gap between the butterflies and the throat of the carbie, thus whistling, and I have also heard something along the lines of it being Dellorto specific due to some difference with the setup running on other cars than what they originally were from- 2Ltr Alfa.

 

Both of these do sound like legit reasons, but if anyone could shed some light into what it may be, that would be great. Being that it appears to be across all four ports, it make me seem less inclined to think that it is a vacuum leak, but yeah, I may be wrong. ;)

 

 

 

Any help would be great :yes:

 

 

Thanks,

 

-Alex

 

 

*edit: Can also get a video if anyone wants to hear.

Edited October 12, 2009 by bLinded_

[philbey]

blinded you know what, I've been meaning to post this exact thing up for a few weeks! It's a bit unnerving when you first hear it, I thought I had a manifold leak at first.

 

Then I realised that it was completely indepenent of engine revs, pitch didn't change and it's purely dependent on the throttle position.

 

I'm guessing it has something to do with the progression jets as it only happens between idle (butterflies closed) and what (fully open).

 

I don't think it's air whistling past the butterflies, otherwise the pitch would change more noticeably.

 

I will check my book tonight, It may be an air bleed that feeds the emulsion tubes specifically for the progression jets and not the primary jets.

[bLinded_]

Haha I've been meaning to post this for months to be honest, but didn't want to sound like a fool asking it :)

 

Yeah I really wasn't sure what the cause of it is, but yes, it is definitely throttle position related. I'm not so worried about it being a problem, but it can get a little annoying cruising around town with it as most of my driving is done with a light foot. (Honestly!) I just wanted some other peoples input as to whether or not it is a common problem, and if there is anything that can be done to stop it :y: Atleast I am not the only one so far with it, so it makes me feel a bit better :y:

 

If you can check your Dellorto book Philbey, that would be greatly appreciated! I used to have a copy of the tuning book when I was building the carbies, but have since returned it to its owner so don't have access to one now.

[Medicine_Man]

Yes, My Dellortos do it to, From what I figured it's the change in Vacuum under load that causes it..

[kiahn]

4age RWD manifolds do it too, its simply the positioning of the air bypass valve which causes it to whistle

on yours I'm guessing it would be the same

i love that whistle, makes it sound like its got a lil 1psi turbo spooling :)

haha

Edited October 12, 2009 by kiahn

[philbey]

Once I figured out that it's not a manifold leak or something I don't even notice it.

 

Or maybe that's because it's either closed. Or open.

[bLinded_]

Alright thanks heaps guys. Makes me feel a fair bit more relaxed, now that I know it is a common, normal issue. Whilst I can put up with it, I was more getting sick of people asking what it was, or was it turbo. But yeah good to know what it is now!

 

Appreciate the fast replies guys too. :) I guess if anyone does have any other light to shed about it, feel free to post it up. More info here the better!

[orangeLJ]

Alot of sidedraught carbies whistle.

 

My old triple SUs has a slight wistle under 50-80% throttle

 

my triple webers do the same at one point, then its just all induction roar.