On rare occasion I run across a QJet with "whistling" primary throttle plates. They scream their heads off at idle and part throttle. I jsut had one today that was doing it. I had just rebuilt it and had even made sure the throttle plates were properly centered in their bores (a regular thing I do whenever I do a rebuild). Figured out the problem dead-bang, though. Something I had done to fix it in the past worked again today, so I'm thinking this may be a semi-common problem with a fairly reliable solution.

The original primary throttle plates had their edges kinda "buggered up." Not bad, but you could tell they had been treated not so good by a previous owner(s). Tiny little nicks and dings here and there. Stuff you could only see up close and personal.

I took off the exposed thread tips of the throttle plate screws with a dremel (factory mashes them flat so you can't unscrew them without doing this) and yanked the plates off. I replaced them with a good set of throttle plates off a junk QJet, and centered them back up in their bores again, same as before.

No more wistling!

Factory QJets on a stock engine often idle with the primaries almost fully closed- a very tiny gap between the throttle plate and the throttle bore. My theory is that the little nicks-n-dings around the ege of the throttle plates disrupted the airflow through that tight area enough to cause a resonance- that high pitched squealing.

Just for QJets? Well, I guess in theory any type of carb could suffer this problem. It's just that the primary throttle plates are very thin on a QJet so it's highly susceptible to this type of problem. Certainly if you've got a Holley or Eddy that likes to "sing" it's worth a close look at the edges of the throttle plates!

 

hmm, could this whistling sound be confused with a vacuum leak?

Also, have you ever had to replace the shaft bushings? are those available to buy ?

 

Not likely to be confused with a vacuum leak. Vacuum leak is like a sucking sound, this kind of whistling is high pitched and can be heard from a block away. EEEEEEEEEEEE!!! The whistling also has no adverse effects on the engine running properly like a vacuum leak would- it's just really annoying.

I have replaced throttle shaft bushings many times, though not personally. I usually outsource it to a local shop, but I am waiting for the special reamer necessary to start doing them myself. The bushing kit is nothing special- about $25 from the shop where I get them. The reamer you need to drill out the worn out throttle shaft holes is $50 and there's really no way to do the job properly without it.

 

hmm, ok, I might be able to borrow a reamer from my work...
I figured the bushing kit would be a very rare part, and next to impossible to obtain... Are the ones you get, a certain common brand? ie edelbrock, niehoff etc ?
I'll look into that, in my unending vacuum leak search...