This Does Not Sound Good ....

[TINKA]

I would not of thought it was your carb as the butterflies are at the base of it and so all the gubbins are not exposed to the vacuum. Sounds like it is whistling in somewhere through a small hole which is why I suspect it has not affected the tick over.

[dave]

Originally Posted by conrod

Right - as I popped the car back in the garage (I've had enough for today!) I watched the vacuum gauge as I revved the engine. As I blip the throttle and the vacuum goes below around 10" Hg the noise stops. It increases in pitch in direct proportion to the rising vacuum when I release the accelerator. The noise increases in pitch to a maximum frequency, which coincides with the maximum vacuum as displayed on the gauge. Tickover vacuum is around 14-16" Hg.

The take-off point for the vacuum gauge is directly from the nearside rear of the inlet manifold.

So this has to be vacuum related. But the vacuum as displayed on the gauge looks normal - if there's a leak (and by the sound of it a large one), why isn't the vacuum reading lower, and why is the tickover still nice and even? In the past when I've forgotten to reattach the vac advance pipe, it's been obvious from an uneven tickover and stalling.

I'm starting to wonder whether rather than there being a leak, something in the vacuum path is vibrating. Could it be an airway in the carb? The carb is a 670 cfm 4150 "Street Avenger" BTW.

But I'm starting to run out of ideas now!

ALAN.
STOP THIS RIGHT NOW!

It was first suggested in post no. 4 that this sounds like a vacuum leak.
After more opinion from different people, the suggestion is still pointing to a vacuum leak.
I know the voices in your head are determined to convince you that is is something else, but this is severely clouding your judgment.
Forget rigging up a flux capacitor or any other such like Edwardian contraption and just get a single piece of 5/8" (Or thereabouts) bore hose and put it to your ear and prod the other end around on your inlet manifold, concentrating on the area around your carb (Where your mechanic (who has seen the car in the flesh) has suggested).
If you do have an air leak that would warrant the removal of the intake manifold then you would be able to find that fault with the hose by listening to the intake runners where then meet the cylinder head.

HTH.
Where's the "Shakes head in disbelief" smiley?

[Bigblock]

Had a problem like this once many years ago when I used to have a Classis Restoration business.

We had a, think it was a ford, dizzy on the side of the block low down, couldn’t find it.
Eventually tracked it down to the vacuum advance unit diaphragm leaking.

The poxy thing had a small rip/split in it. It used to whistle and make a farty rasping noise.

Just clutching at straws for you now.

[wilf]

Does this engine have a proper "anti walk" cam button?

[Vidge]

You have a vac leak or a very small annoying man has been sucked into the carb and is blowing on a blade of grass held in his cupped hands.

I've got a pair of holly carb's sitting in the garage at the moment, one's jetted for 650cfm the other 750 also there's a refurb'd edelbrock intake manifold if you want to borrow them somehow.

I'd check the carb baseplate first and then the intake manifold replace the gaskets and sealant then bolt it back together.

Good luck with it

[conrod]

Originally Posted by dave

ALAN.
STOP THIS RIGHT NOW!

It was first suggested in post no. 4 that this sounds like a vacuum leak.
After more opinion from different people, the suggestion is still pointing to a vacuum leak.
I know the voices in your head are determined to convince you that is is something else, but this is severely clouding your judgment.
Forget rigging up a flux capacitor or any other such like Edwardian contraption and just get a single piece of 5/8" (Or thereabouts) bore hose and put it to your ear and prod the other end around on your inlet manifold, concentrating on the area around your carb (Where your mechanic (who has seen the car in the flesh) has suggested).
If you do have an air leak that would warrant the removal of the intake manifold then you would be able to find that fault with the hose by listening to the intake runners where then meet the cylinder head.

HTH.
Where's the "Shakes head in disbelief" smiley?

Dave,

It's just frustration setting in Dave! Now the sun's shining I just want to find and fix the fault, then make the most of the good weather while it lasts!

As per an earlier post today, I tried the hose technique (well OK, my variant of it ), but drew a blank. It seems incredible that a noise so loud should prove so hard to nail down, and there's been two of us searching today! But I'll put my flux capacitor stethoscope mod back in the De Lorean and cannibalise my garden hose tomorrow!

Plus I think I'm developing tinnitus now - my ears are still ringing ....

[wilf]

Jesus Alan, if it takes this much effort to track down a noise that loud on an engine as simple as this, how the hell will you manage when the missus tells you "the cars making a funny noise dear......."?

Anyway, it's rain for tomorrow...............

[conrod]

Originally Posted by wilf

Does this engine have a proper "anti walk" cam button?

Wilf,

I only know that it's not too lumpy Crane cam! If your question relates to end float and possible stress on the distributor drive, the teeth on the gear looked fine when I removed the dizzy.

The engine was built by Mike Huddart, so if this is a standard feature of a tuned SBC engine, I'm sure Mike would have fitted it

Having striven and failed to identify the source of a leak, we did start to wonder whether there could be a mechanical cause which tallied with the noise disappearing in the transition from idle or overrun to acceleration. It was at that point we convinced ourselves the noise was emanating from the dizzy. A moment of madness ......

But the direct correlation between the pitch of the noise and the vacuum level in the inlet manifold is definitely pointing to a vacuum leak - you'll be pleased to hear Dave!!!

The hunt for the elusive leak will continue tomorrow afternoon. I'm just glad I've got my neighbour working on this too - a problem shared and all that. He's a genius with old car and bike engines - so I take some comfort from the fact that this one has him stumped at the moment too!

[conrod]

Originally Posted by wilf

Jesus Alan, if it takes this much effort to track down a noise that loud on an engine as simple as this, how the hell will you manage when the missus tells you "the cars making a funny noise dear......."?

Anyway, it's rain for tomorrow...............

Simple - I'll plead deafnesss - "can't hear you dear"!!!

The irony is that every time we start the engine to apply logical analysis, we simply can't hear ourselves think. That's why I wheeled the car out of the garage today - the noise in a confined single garage was driving me nuts. That plus the petrol fumes from removing the carb twice today.

As I mentioned earlier, I take comfort from the fact that a professional mechanic can't pinpoint the source either! Otherwise I might be thinking about calling the men in white coats myself. Just to get away from that bl00dy noise!!!

[maurice butler]

heres a trick from the colonies. get an lpg bottle with regulator and hose - the one off the bbq normally does well.

From this point you need to be careful so you don't blow your self up.

turn the bottle partly on so there is a small amount of lpg coming out of the hose. move the hose around the suspected area of the leak with engine running. when the revs at idle climb you are close

Maurice