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Discussion Starter #1
I have an issue that's driving me crazy, so any help is very much appreciated.

The issue....1972 450SL, recently overhauled, so all the ignition, timing, and fuel injection settings are supposedly correct and working A-OK...per my mechanic. The engine runs perfect above 1200 RPMs. At exactly 1000 RPMs, whether the engine is hot or cold, it misfires badly...sounding and feeling like 2 or more cylinders are missing. Around 1200 RPMs, it levels out and runs A-OK. My mechanic suspects the ECU computer, so I bought a used one on Ebay, and the results are the same.

My question is...does this sound like a ECU problem? If not, what other things should we consider? He has replaced all the ignition items (points, trigger points, spark plugs, plug wires) and has verified the fuel injectors are working. Help!
 

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R.I.P Always Remembered
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IMHO either the valve timing or ignition timing is out of wack. At higher rpm the timing becomes advance and works well but when the revs drop and timing returns to normal it will misfire. Re check the valve timing and then look at ign timing.
 

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Discussion Starter #3
Thanks Eric. My mechanic (who did the engine overhaul) says all the timings are where they are supposed to be. However, I will pass your advice on to him for additional follow-up. The fact that it idles A-OK at 800RPM, misfires at 1000RPM, then smooths out again at 1200RPM sure sounds like something to do with timing advance movement that I believe takes place in the distributor. A valve job was part of the overhaul, but could valve timing cause the problem described?
 

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R.I.P Always Remembered
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Does this engine have two distributors? Sorry by 72 was a lot of years ago and I am old.:biggrin: If so check inside the caps for tracking and check the back of the rotor arms for traces of tracking also. If it idles OK and is OK at 1200 rpm then the chances are it is not the valve timing but shorting in the electrical system under first load, ie inside the dist caps.

Assuming, that is, that the caps and rotors are not new.
 

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Discussion Starter #5
No, it only has one distributor. The cap and rotor are relatively new, but I will check for the tracking you reference. By tracking, I assume you mean worn spots?
 

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Discussion Starter #8
Due to lack of responses, I will close this thread. I have isolated the problem to an issue with the amount of fuel being released by one of the pairs of cylinders controlled by the Trigger Points or the Throttle Position Switch....not sure which yet. The issue is complicated by the fact that it only occurs between 1000-1200 RPMs. The problem is not with firing of the spark plugs nor with normal operation of the fuel injectors. If anyone is interested in learning more about this, I have opened a new thread on the BenzWorld forum and have received a lot of help there.
 
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