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Old 11-15-2001, 03:05 PM
Posts: n/a
Angry Running Rich!!!! 280SEC

I just bought a 69 280SEC, looks to have been a very well taken care of car. I was told it runs very well, but closer inspection proved it to be a very rich running car. Gas in the oil , blowing smoke on acceleration ect. Timing is set at 4 degrees ATDC. At speed it runs well, but use a great (10mpg) amount of gas. At idle, the engine surges , rpm's fluctuate between 800 and 950. What adjustments can I make to lean her out? I do not think this is an ignition problem. The car did have a coil contact problem but I have fixed it. New plugs, wire set, ect.


61 220SEC
69 280SE
72 250C
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Old 11-15-2001, 03:16 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784
The surging idle tells me that the first thing to check
for is a leaking cold start valve. Fairly common - could just need a cleaning.
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Old 11-15-2001, 03:32 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The most common cause of rich running is a dead or stuck warm-up thermostat. It is very easy to check not necessarily that easy to fix.

At the rear of the pump is a metal air filter. Remove the filter. With the engine idling cold place ones finger over the hole. It should kill the motor. Now try it again with the engine hot. While suction will be felt there should be no change in rpm..

In the tube (with water hoses attached) above the filter is the water warmed t-stat. It pushes a plunger into the pump which does two things. One it controls air added for fast idle cold and two it enriches the govering mechanism. The mixture is harder to measure but the airflow is easy. Same action does both. If there is significant drop in engine speed hot then the choke correction mixture is also off.

The whole arrangement disassembles quite easy and the piston inside is often frozen (usually in the cold setting). Just pay attention to all shims. They are in with the t-stat and beneath the assembly. Don't loose any untill we get to the bottom of this.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 11-28-2001, 03:42 PM
Posts: n/a
Back on the running rich problem

Sorry I did not reply to either of you guys. The information given was helpful. The cold start valve was clean and I tested it. Works just fine. I wanted to get a replacement seal and valve kit for good measure but I could not find one. MB would only sell the complete valve for $800 bucks! All as well for what I have works and does not appear to be leaking.

The thermostat for the injection pump is another story. The pin was stuck (about half way out), once it was freed the pin fell out. I have ordered a new one. I don't really under stand the whole arrangement. The pin has a bulb on the end of it so when the thermostat is heated the pin extends. The bulb prevents the pin from traveling through the tunnel. In the tunnel There is a keeper, which makes me think that the insert in the tunnel also moves, but is now frozen. Is this correct? Or is the pin supposed to travel all the way through?

Thanks for the help,

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