I have a 73 450 sl mileage unknown.I have a very bad miss when idling and driving.I have replaced the plugs,wires,cap and rotor and also cleaned the injector points.I attemted to set the timing but the soap-stone mark will not stay stationary.The car is also running rich.It is not burning any oil and there is no antifreeze in the oil.Would my next step be to check the valves?

Thanks in advance

Mike,

You might want to check the timing chain and rails for breakage/stretch. Retarded cam timing can lead to a rough idle (and more serious consequences if the rails have broken!) Good luck!


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Rgds,
Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'80 300SD
'85 380SE
'89 420SEL
'93 300E 2.8
'74 Jensen Interceptor Mk.III
'81 DeLorean DMC12
'85 BMW 745i Turbo

Since you of course have a well maitained auto, there should be no need to adjust valves. MB actually wanted them adjusted every 10k miles in 73. If you let them go for maybe 70,000 miles between adjustments you might get an idle problem or eventually (although I have never seen a V8 abused badly enough) burn a valve.

You need to deal with your timing problem first as the EFI V8s were so sensitive to manifold vacuum for fuel control. I would disconnect all vacuum lines to the distributor till you get a stable light. Without any vacuum control you should have at least 10 degrees advance at idle. You should have 15-17in manifold vacuum.

If you have misfires it is not likely a valve problem or a mixture problem unless you are fouling plugs. Identify the offending cylinder by pulling injector wires. This kills the cylinder and the resulting misfire should occur evenly on each cylinder. The ones that don't make a difference aren't running properly.

Deal with your vacuum/timing problem and get rid of the misfires and I might tell you how to adjust the mixture.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician