panic

It's sometimes now I'm struggling to understand WTF is causing a scaring hard hitting noise coming from the intake area of my 406ci 575HP engine...it sounds a bit like if there was a hole in the headers, but there is none, I've also changed the headers gaskets, just in case.

It appears when accelerating, at idle it disappears. The overall performance is gone, the car runs at a fraction of its usual speed.

I've opened the valve cover, the whole hardware there is ok, BTW about a month ago I've adjusted the valves slash and changed a rocker, everything seemed by the numbers.

The noise started a sunny sunday while returning from a short journey to the beach.

The engine has always enjoyed fresh oil and clean oil filters from its very beginning.

My personal opinion is that there might be a valve that is broken or out of place and that the detonation in a cilinder goes out from the carb filter instead then going the exhaust path, could it be? Does it sound too naive?

I'd appreciate your comments, thanks a lot in advance.

Postal123

I'm no guru with performance engines, but based on your description it sounds like a backfire. Have you done anything that might affect your timing? I'd think a problematic intake valve could cause something like this also.

You mentioned its fine at idle, but occurs during acceleration. Is this true all the time? At what RPM range does it begin and does it continue in the upper RPM ranges?

Hopefully its something simple that you're overlooking.

doorgunner

Another member had nearly the same problem a day or two ago....he found a loose rocker arm was causing back-pressure to pop inside the intake manifold.

That may not be your problem since you already checked the valve train.

Do you have a mechanic's stethoscope to listen to the engine and narrow down the area where the noise is coming from.

If you have no stethoscope, use a vacuum hose about 4 feet long with a 3/8" I.D. Hold one end of the hose tightly to your ear and hold the other end very close to the engine (1/8" from touching metal). See if you can find the spot where the noise is the loudest.

Let us know what you discover.

Haggisbash

Hmm, if the performance is gone are you sure the distributor hasn't shifted? Timing too far advanced?

panic

Timing has been curved by the engine builder from the very start and has never been touched, it always worked fine since day one.

Always happens when accelerating, iddle is always clear. It happen from 3000/3500 rpms when accelerating suddenly, also from standing in neutral.

panic

You mean a broken distributor?

I don't think it's a problema of timing, it's not a knocking sound as if the engine was too much advanced.

The sound is more likely a back fire in a valve, too loud for being anything else.

The problema is WTH is causing this BS, the engine is a state of the art hardware, always problema free, always driven with velvet gloves...

Cavu2u

Any new, or unusual smoke coming from the exhaust?

panic

No, nothing...

Cavu2u

Well, that's good news. Eliminates a few things anyway.

Have you checked the engine timing since the problem developed?

Steve

panic

No, but I will next.

Anyway, how could timing move from where it successfully has been for years? A broken distributor maybe?

JimLentz

The hold down bolt for the distributor could have come loose.

Postal123

The reason for my first response was that what you described sounded like a bit of combustion in the intake system. If advanced too much this can cause 'your stuff' to light before the intake valve is finished closing. It sounds like its acting up under acceleration and about the time the timing should be all in...maybe too much total advance?

You also mentioned that you recently adjusted valves and changed a single rocker. Did your problem begin immediately after your last maintenance?

With overall power being down quite a bit...I'd still look at timing first because it's easiest to confirm. I'd think it could also be lash too tight across the board or a single cylinder acting up.

panic

No, the engine worked well for a few hundreds miles before showing the malfunction...

Today I'll check the distributor...

Tampa Jerry

Put a vacuum gauge on the engine from an intake manifold port and report back what you find. There are a few websites that explain what the readings mean. A fluttering reading will usually point to a valve train problem. Jerry

panic

Well, here is a summary:

I've checked the rocker arms and the valve lash....OK
I've checked the engine compression....OK
I've checked the distributor.....no sign of problems whatsoever.
I've checked the advance...is correct.
I've changed the spark plugs (just in case)....

Still, this fuccking noise is killing me.

Here are three short videos, you please listen when I push the throttle...like a noise coming from a cave, actually it comes from the intake...

'75

You could try removing 1 spark plug wire at a time to see if the noise changes. That may identify which cylinder is causing it.

Cavu2u

What type rocker arms do you have in there? Stock, roller type?
Did you check your pushrods for any cracks, bends, chips off the ends?

Z06FEVER

Sounds like it could be incorrect valve adjustment, out of mechanical time, bent pushrod or a lobe going bad on the cam, etc.... I believe it is definitely in the valvetrain.

drkness01

Rocker replaced....why?

sounds like a wiped exhaust lobe

panic

When I adjusted the valve lash found one arm slightly worn out so I changed all of them.

I supposed that for this kind of (race) engines with about 10.000 miles on the clock and never rebuilt a worn arm was nothing to worry about...