Help I think I'm running out of guesses. The car is a '74 450SL (107.044) with 150K miles. It has developed a serious mis-fire that at first seemed to be very random and intermittent. Now, it only does it on the first trip of the day. Colder weather may affect it a little, but it also stumbles and barely runs after having been in the garage all night. The rough condition is present at idle and at all other speeds. When it's doing it, the car will barely get to 50 mph, but there are intermittent surges where it seems to "catch", and the car will take off with good power for a second or two.

The car starts easily and runs fine for about the first mile. Then it starts running really rough and is possibly blowing a little smoke, like it is running really rich or possibly not getting a consistent spark. It will continue to do that for several miles and will continue even if a couple of stops (ignition off) are made. When I get back in the car at noon, it will run fine for the rest of the day.

I have checked the vacuum distributor retard circuit; it checks out and the car does the same thing with the vacuum line disconnected and plugged. I have gone through the check sequence on the fuel injection control box. I suspected the throttle valve switch and replaced it with no improvement. I replaced the high tension lead. I obviously checked the timing and dwell after I replaced the points and condenser. I checked the fuel tank venting system, thinking that the tank was not being vented and was developing a vacuum on cool mornings; it did not improve with the pressure value disconnected and the vent line seems clear. I put in gas line anti-freeze, suspecting water in the tank. I checked fuel pressure in the fuel ring, which was at 2 bar. The fuel ring held pressure for a long time, so I don't there are leaks.

In the past, I checked the electronic ignition box, but I haven't recently, so that could still be a suspect. I was suspecting the fuel system, but now I am thinking ignition. Ideas?

Thank you for your help.


Jim Martin
Durango, CO