PeachParts Mercedes-Benz Forum - View Single Post - 73 450SL purrs then runs rough after 5-10 min

stevebfl · #5 06-14-2003, 02:23 PM

The adjustment on the computer only affects idle. The way to set this system up is to get your timing high (at least 5 degrees above spec don't get fooled by the vacuum retard - the spec intends you to be retarded). I would remove the vac retard line and set it to 10 degrees BTDC (maybe more - go for 17in manifold vacuum after the fuel is optimum).

Now to adjust the mixture: disconnect the throttle switch. This removes the computers idle comp circuit (so one can work on just the main system). To adjust one must open the black cap on the manifold pressure sensor and use a 4mm allen wrench. I do it on a dyno so that I can check it at various loading but it will work OK to set the CO to 1% at 2000rpms. After setting reconnect the throttle switch and adjust the control unit to best idle : usually 2-3% CO. If you wish to adjust this system you must do it with an exhaust gas analyzer. I have probably done it a thousand times maybe ten and I would only do it by ear in a pinch but since you are going to do it that way anyway, use the above technique.

Without a dyno one will only be sure of proper adjustment under all loading if the manifold pressure sensor is still at full range. Since its action is based on an evacuated bellows it looses its range over time. A functional test can be done with a scope by measuring the injection pulsewidth. Hook scope to injector and turn key on engine off. By hand, move the throttle. There will be 20 firings one at a time as one moves the throttle to full. Only two injectors will pulse (the ones that correspond to the trigger point closed at the moment). Hook to one of these injectors and view the pulse on the scope. At the moment there is zero manifold vacuum (not running) so the pulsewidth is maximum - probably 7-10ms. Pull a vacuum on the manifold sensor and the pulsewidth should go down. At 17in it will be about 2.5-3ms.