1974 Mercedes 280C Idle Speed

[grubby65]

I'm having an issue with the idle speed on my 1974 280C. Over the last year I have had to continually adjust the stop to keep the idle speed up. This is something that has happened very gradually. Last night I checked the timing which was OK and I also sprayed some carburetor cleaner around to check for vacuum leaks but found nothing. I was wondering if anyone has a suggestion.

[Frank Reiner]

Federal or California version?

[Graham]

Originally Posted by grubby65

I'm having an issue with the idle speed on my 1974 280C. Over the last year I have had to continually adjust the stop to keep the idle speed up. This is something that has happened very gradually. Last night I checked the timing which was OK and I also sprayed some carburetor cleaner around to check for vacuum leaks but found nothing. I was wondering if anyone has a suggestion.

Does that model have Djet fuel injection or carb?

[Stretch]

With out the E I would expect a carb

[Frank Reiner]

Originally Posted by Graham

Does that model have Djet fuel injection or carb?

Originally Posted by Stretch

With out the E I would expect a carb

All the US W114/M110 versions used the Solex 4-barrel carb.
'74 California version is unique; one year, one state.

[Stretch]

Originally Posted by Frank Reiner

All the US W114/M110 versions used the Solex 4-barrel carb.
'74 California version is unique; one year, one state.

Great stuff - have you got any advice for the OP?

[Graham]

Originally Posted by Stretch

Great stuff - have you got any advice for the OP?

Haven't had a car with carbs for quite some time.

Maybe jets getting blocked, fuel pressure low or maybe just fuel filters need replacing?

Just guessing with no knowledge of car - Frank should be able to help.

[Frank Reiner]

Originally Posted by Stretch

Great stuff - have you got any advice for the OP?

Certainly, when the OP specifies the version.

[grubby65]

The car is a Federal Version and the Solex Carb. It has a Petronix conversion to the ignition system. The vacuum advance is unhooked. I run 10 degrees initial advance and all in the advance is right at 40 degrees. The car runs fine. It has a slight hesitation right before the secondaries kick in, but nothing terrible. I run 91 octane (non-ethanol) gas. I check the fuel mileage at every fill-up and it routinely is between 17-18 mpg. I live in rural Nebraska and drive it to work 50 miles round trip daily. There is very little time spent in city traffic. Just for fun I took to the cover off the air cleaner to make sure I didn't have a plugged filter or an air restriction somewhere and the idle speed remained the same.

[Frank Reiner]

Originally Posted by grubby65

I'm having an issue with the idle speed on my 1974 280C. Over the last year I have had to continually adjust the stop to keep the idle speed up. This is something that has happened very gradually. Last night I checked the timing which was OK and I also sprayed some carburetor cleaner around to check for vacuum leaks but found nothing. I was wondering if anyone has a suggestion.

Originally Posted by grubby65

The car is a Federal Version and the Solex Carb. It has a Petronix conversion to the ignition system. The vacuum advance is unhooked. I run 10 degrees initial advance and all in the advance is right at 40 degrees. The car runs fine. It has a slight hesitation right before the secondaries kick in, but nothing terrible. I run 91 octane (non-ethanol) gas. I check the fuel mileage at every fill-up and it routinely is between 17-18 mpg. I live in rural Nebraska and drive it to work 50 miles round trip daily. There is very little time spent in city traffic. Just for fun I took to the cover off the air cleaner to make sure I didn't have a plugged filter or an air restriction somewhere and the idle speed remained the same.

The Federal version makes things a bit easier; the gradual change in idle speed suggests that the first step is to adjust the valves. Cold .004/.010".

Subsequent to setting the valves, there are a few other things that can affect idle speed:
Leaks from the EGR system.
Leakage or blockage of the crankcase vent orifice/valve.
Incorrect or varying float level.

Can you post some pics of the carb with the air cleaner removed? To ensure that it is the OE 1974 Fed type.

[kraakevik]

The spring-loaded vacuum-controlled diaphragm in the lower left of the photo should regulate the throttle opening whenever the car is in Drive or when accessories are operating.

This should keep the rpms in the 600-800 range when engaged. Without vacuum applied the throttle will be pulled back to run at 1800 rpm or so.

The only time the throttle-stop screw should touch the throttle arm is when the engine is under no load at all--such as in Park or Neutral.



Tim Kraakevik
kraakevik@voyager.net

[grubby65]

Originally Posted by Frank Reiner

The Federal version makes things a bit easier; the gradual change in idle speed suggests that the first step is to adjust the valves. Cold .004/.010".

Subsequent to setting the valves, there are a few other things that can affect idle speed:
Leaks from the EGR system.
Leakage or blockage of the crankcase vent orifice/valve.
Incorrect or varying float level.

Can you post some pics of the carb with the air cleaner removed? To ensure that it is the OE 1974 Fed type.

Here are some pictures of the Carubretor

[grubby65]

I needed to change the valve cover gasket so I decided to check the valves while I had it off. The majority of the intake valves were between .005"-.006" and the exhaust valves were between .012"-.014". Is the clearance off enough to readjust? Also, what in the world type of tools do you need to adjust the valves. I'm not above making tools, but I was trying to figure out what kind of crazy looking crow's foot I would need to make.

[Frank Reiner]

Originally Posted by grubby65

I needed to change the valve cover gasket so I decided to check the valves while I had it off. The majority of the intake valves were between .005"-.006" and the exhaust valves were between .012"-.014". Is the clearance off enough to readjust? Also, what in the world type of tools do you need to adjust the valves. I'm not above making tools, but I was trying to figure out what kind of crazy looking crow's foot I would need to make.

Here's pics of the usual tool for that engine.
The concern about valve clearances was that they not be too tight; as they are now they will not affect idle.

All about the carburetor appears to be original. There is a specific sequence for setting idle speed in the OEM manual:

• Engine at operating temp. Trans in N/P. Vacuum governor rod/screw retracted, not touching throttle lever.

• By means of the idle stop screw (small screw on carb body, set @ ~45deg) set idle speed to 750-900 RPM, touch up mixture screws to achieve smooth running.

• Pull off hose from vacuum governor.

• Set high idle speed to 1200-1400 RPM (counter-hold screw when loosening & tightening locknut).

• Replace hose on vacuum governor. Place trans in D.

• By means of the spring adjusting nut on vacuum governor, set idle to 600-700 RPM in gear.

Run through this speed setting sequence before getting into float level matters. Sad to say, fuel pressure and float level control are often not as good as we would like to see, but that can be remedied.

[grubby65]

Originally Posted by Frank Reiner

Here's pics of the usual tool for that engine.
The concern about valve clearances was that they not be too tight; as they are now they will not affect idle.

All about the carburetor appears to be original. There is a specific sequence for setting idle speed in the OEM manual:

• Engine at operating temp. Trans in N/P. Vacuum governor rod/screw retracted, not touching throttle lever.

• By means of the idle stop screw (small screw on carb body, set @ ~45deg) set idle speed to 750-900 RPM, touch up mixture screws to achieve smooth running.

• Pull off hose from vacuum governor.

• Set high idle speed to 1200-1400 RPM (counter-hold screw when loosening & tightening locknut).

• Replace hose on vacuum governor. Place trans in D.

• By means of the spring adjusting nut on vacuum governor, set idle to 600-700 RPM in gear.

Run through this speed setting sequence before getting into float level matters. Sad to say, fuel pressure and float level control are often not as good as we would like to see, but that can be remedied.

Thanks for the help. I will try these settings tonight. What is the device called between the fuel line and the carburetor, I think the third line is a return. I need to replace it because the brass tube between the body and the carburetor came apart and dumped fuel all over the engine.