Various engine noises and missing on part throttle

[lamakdaddy]

I am the proud owner of a 1971 280sl manual, US spec with AC

The car performs well and has strong acceleration but seems to hesitiate on part throttle and is noisey.  She idles at approx 800 rpm but hunts slightly (i went through the linkage tour btw)

I have noticed the following

1.  I can hear quite a loud hissing from around air filter housing when the car is running. I cannot find any obvious air leaks. Do they all make a hissing noise from this area.

2. I hear a tapping noise- very much like a sewing machine when acclerating. Its engine speed dependent and starts from around 2000rpm and increases as the engine is accelrated.  It comes from past the bulkhead area and the engine is not noisey when listened to from outside. I thought it was a cracked / blowing manifold but its too metallic in nature.   It sounds like tappets but the mechanic says that they all make this noise - I cannot imagine a new 280sl making this noise.

3. On the throttle linkage I have 2 electromechanic devices- one has electical terminals but doesnt appear to be attached to any inputs (CIS?) and the other looks like a vaccum valve but is also not attached to anything.  Should they be attached to an input of some description?

The car otherwise runs very strongly with excellent accelration etc.

Could someone please check whether these sounds appear on their cars?

[Cees Klumper]

These kinds of things are always difficult 'in writing'. It would be great if you could post a video or at least MP3 of the sounds so we can compare with our own experiences.

On the air intake, I did notice that when I ran the car once briefly with the air filter off, the hissing sound was very strong. But with everything in place as it should be, I can't say I hear any particularly strong air intake noise.

On the mechanical tapping sound, try taking off the valve cover to see whether the posts that the valve cover bolts go into (so the 'tower brackets' that allow you to bolt down the cover) may have gotten bent by someone overtightening those bolts. Smetimes they bend so far that they start touching the camshaft, producing a metallic noise. Also check the valve clearances and just make sure there isn't a broken valve soring or something else clearly amiss there. The fuel injection pumps tend to be a bit noisy.

[w113dude]

I totally agree with Cees, once every thing is tuned right that hissing noise will reduce but not completely go away.