Pre-Ignition/Pinging/Detonation ?

[Ben]

I'm not sure how long this problem has been
present, as I listen to the radio all the time. I seem to be hearing a sound
like Pre-Ignition but the curious thing is its very faint and I only hear it
when cold and under gentle acceleration. This kind of goes against the
grain, normally you'd get it when hot and labouring the engine.It disappears
totally when warm ? The timing is set correctly, I'm sure of that, but I
retarded it anyway which made no difference. Any ideas ? Could something
else sound like that ?

Regards,
Ben


Regards,
Ben in Ireland.
'64 230SL 4sp.

[hands_aus]

mixture too lean when warming up?

Air slide valve in inj pump moving freely?

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Ben]

Actually as an update, I've just driven it again and it happens when warm too, not quite as noticeable though, but always present !

Air Slide valve perfect and I set the mixture with a meter to 4.5CO. when warm. It doesn't smell like its anything but rich !!

I'm gonna check all the settings again and read the plugs !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[graphic66]

This is a long shot, but I have heard plug wires make noise before when they arc to ground. A couple of ways to check for leaking plug wires are, look at night under the hood for arcs and listen to your radio on AM, you can hear a pop pop as the wire arcs in the radio.

[n/a]

Piston slap? Much more prevalent under light throttle, and decreases with increasing engine temp. From my experience with my '83 W-123 230TE Benzes are particularly prone to noise with the slightest excess clearance.

Neil Rote, Melbourne Australia
Unrestored '64

[Ben]

Yeah actually my leads are kinda old looking. I might do that later tonight !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[hands_aus]

Ben,

a silly question... do you hear the noise when the radio is turned off?



Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Ben]

quote:Ben,

a silly question... do you hear the noise when the radio is turned off?

Yeah....why ?

I tried changing the timing, havent got that strobelight yet, retarded it about 5 deg, possibly more until it runs horribly. Anyway it still makes the noise !!

I dont know for definite if it is pre-ignition, cant tell for definite, but I am going to try an Octane booster to see what happens !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Malc]

Ben,
This might sound daft but are you sure it's the engine?
Could it be alternator or even the water pump making "tinkling" noises
even something like the heater fan hitting the casing, but only when its cold!
Just some "off the wall" thoughts
Malc
PS when you do find out what it is let us all know!

[Benz Dr.]

Most likely piston knock or a worn piston pin bushing. Valve lash noise gets worse and louder with inctreasing temps.

Daniel G Caron

[Ben]

Mmm I wondered when someone would suggest that, however since everything, pistons, pin bushes, in fact the complete bottom end is only 1 year old I'd be surprised if thats the cause !

If it IS something like that then I pressume if I pulled a plug off each cyclinder and drove it,  the noise should change, assuming EACH cylinder wasnt in trouble !

Malc......I havent really tried to find the cause yet but it could be one of the other components !

Keep you posted !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Malc]

Ben,

Could the injection pump be making noises???
If you can get a stethoscope to listen to various parts of the engine that will help.

However difficult to imagine a problem in a 1 year old engine...

Maybe you could try a compression test on each cylinder, with the engine cold and then hot. Hopefully they would all be within 10-15psi of each other.

Another thing to try is take all the plugs out and if possible rock each piston across TDC for that piston. If you can place a rod down each bore through the spark plug hole and measure the degrees of turn of the crank, feeling for when the piston stop travelling upwards, stays "still" at TDC, and the starts to drop (two person job)

if one "stays still" over TDC markedly more that the others that's the one with a worn little end bush. It might not be possible to feel this work if the wear is small, but might be worth a try

As to piston slap I was under the impression that is cause by worn or broken rings, which may show up in the compression test.

I would concider doing this before ripping the engine apart

On the other had if the car is running fine and everything else appears to be ok I would live with it
Malc

[Ricardo]

Hey Ben
A couple of other possibilities...Joe Alexander once talked about a noise that was caused by the valve cover being too tight, causing the bracket that it is bolted to, to distort and then the cam rubs against it.
Another thought...I had a slightly bent dist. shaft in a chev 350, thing ran fine, except for a light knocking sound which went away with a little side pressure on the dist. cap.
Ricardo

[hands_aus]

I thought if it was there only while the radio was on it might be electrical type interference.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Cees Klumper]

I would definitely check what Ricardo mentions: the valve cover brackets, see whether they touch the camshaft.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[Ben]

Well thanks but since this noise is not there when idling, only when accelerating between 1500-3000RPM it couldn't really be the cam bracket issue !

I am going out the door right now to have my timing checked again with a strobe light !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Ben]

Okay a small update.........I had reported that all my ignition settings were spot on.....they were not !

The dwell was over 50deg instead of 39-41, and the timing appeared to be all over the place !

It was jumping around like crazy ! My idle had been a bit strange but nothing anyone would worry too much about. Upon closewr examination, when trying to reset the points I noticed that the baseplate was very loose, you could rattle the points all over the place !

There is a little tag which the rear of the points can slip under, and also a little ballbearing retainer and by adjusting both I got it tighter. There is still too much movement so I will be on the line to Dan for one of his distributor rebuild kits and take it from there. It does seem to idle and pull much cleaner now but I still have the noise............it has been suppressed to the extent that you really have to listen for it.....but i neeed to get to the bottom of this asap !

Keep you all posetd !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Ben]

If the springs in the dizzie are weak or gone what are the symptoms ?

I know the centrifugal action will advance the timing earlier than usual, and if the vacuum pull rod is set incorrectly the same can happen more or less.

Basically as far as I can tell, with a very basic timing light, my settings are correct but I still feel I'm getting this pinging noise. Its only under full throttle at about 1800-3000RPM after which the car flies and obviously full advance isn't a problem here. There are no percievable flat spots either at any revs !

Is there an absolute spec somewhere that will tell me what I should have at say 1500,2000,2500 etc. ?

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Naj ✝︎]

12-19 Degrees at 1500 rpm w/o vacuum.
naj

65 230SL
68 280SL

[A Dalton]

Ben
 Pull the dist, remove the plate and take a look... they sit right there .  
A trained guy can tell by simply turning the rotor by feel , but a look/see will be evident if you have broken springs/arms/ or weight mount post.
 Usually, rapid , low rpm advance [ which is suspect on yours], is one spring has broken, usually at the mounting eyelet...
 ..That is where I would start, anyway..

[Ben]

Thanks Naj and thanks A.Dalton !

I drove it home yesterday and parked on the driveway then pulled the dizzie and decided to take it apart anyway. When I opened the front door there was Dan's rebuild kit sitting there, thanks Dan.

Talk about perfect timing (no pun intended), its a comprehensive kit too ! So I striped the unit, but saw nothing really wrong along the way so far except the following. The screw that holds the upper plate wasn't very tight and neither was the little ball bearing holder so that plate could move quite a bit ( should it be shimmed up to a certain height?) Also the Bosch points I have dont sit very well on the post. If I push them down the two contact points misalign and almost miss eachother ( should there be washers fitted to the post? )

Having removed all this and seeing that the fibre washer was in good shape and bother springs were attached I became a little dissilusioned. The drive dofg on the end will come off this morning but i noticed a lot od up/down movement, which I'm sure the spring in the dizzie housing will prevent, but I may add washers here. This is this drive can move sideways very easily on its roll pin, is that normal ?

Anyway I'll go through the motions and post photos later !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[Benz Dr.]

The plate that the points sits on has to be tight and yet still be able to move ( rotate ) freely. The small ball bearing has a cup shaped spring which holds it in place. The arm of the spring is held by a very small screw and you can undo this a bit and move the arm down which will put more pressure on the plate.
On the other side of the plate there is a small arm which also holds the plate down. Sometimes this arm is too high and needs to be knocked down a bit so the plate will be held tighter. You don't want it so tight that it can't move - just take all the play out of it.
When I do a rebuild I have every part out of the distributor and each piece is cleaned and inspected. I use a small amount of light grease between the base and the plate the points sit on. The points should line up with each other and you can bend the point arm up or down if needed.

The advance springs almost never come off or break but they can get weak. If someone uses the wrong lenght of screw to hold the condensor or any other part on the outside of the distributor body it can actually hit the arms that hold the springs on and bend them very badly. This I've seen quite a few times.
 The arms can be straightened but for the most part I consider them damaged and they should be replaced. They seem to not hold the springs as well and tend to move back towards how they were bent over time.
The advance curve is more a function of the weight of the flywheights. Light ones have a slow advance and it tops out at a higher RPM  while heavy ones ( like in 051 ) top out quickly.
 On some units the flyweights will be 2 different sizes and this will produce 2 different advance curves. The first one will be similar to most other units and then it will level off with another advance higher up in the RPM range - usually around 4,000 RPM.
 048 vacuum advance and early 450SL's have this feature.

Daniel G Caron