1971 280SL US model running lean

[Pag280]

Hi, I am new to this website and I have no experience regarding this car, so I am going to need your precious advice. I am in the process of re-commissioning a 1971 280SL which has been sitting for long time. I drained the fuel which was very dirty and replaced the fuel filter which was badly clogged. I put new fuel and hooked a pressure gauge at the cold start valve fuel line and the reading was 0.75 bar. I also tested fuel flow returning from Injection pump and I got 0.9lt in 15 secs. I replaced the spark plugs with a new set of NGK BP6ES. Distributor is an electronic 123 system so the transistor system is bypassed. I removed the warm running device and tested the thermostat in hot water until it boiled but there is no movement, so I installed it again until I can get a new one. I adjusted the linkages as per manual and adjusted the intake butterfly to stop exactly before it binds. I also performed a compression test and got an average of 160psi.
The car starts with a bit of cranking and idles pretty high at 1500rpm. I tried to adjust from the air screw but there is no significant change. When I did the split linkage test I started by pressing the IP lever and the revs kept increasing while when I open the intake butterfly slightly the engine died, so it is definitely running lean. I would like to check the ignition timing but I cannot get it to idle slower, so I am a bit stuck. Can you please advice?

[Peter h]

Did you let it run longer to get out of the cold start?
Otherwise there is no reasonable result.
Peter

[mrfatboy]

With the WRD thermostat bulb non functioning the FIP will never adjust to its final warm running state. The FIP will always think the engine is cold.  I would advise getting a working thermostat bulb before proceeding.

Once you get the WRD working properly you can begin the other adjustments.

[Pag280]

I have already ordered a list of parts including thermostat. I will get back to you when after fitting them. Many thanks!

[Shvegel]

As the others have stated your warm up thermostat is holding you back.  The warm up thermostat does two things the engine needs when it is cold.  It adds more air and it adds more fuel.  As the engine warms and the thermostat pin extends it not only reduces the amount of fuel but also cuts off the additional air supply.  The reason you couldn't get the idle down is there is still a lot of air running from the the little air filter on the back of the pump through the tube running across the back of the engine and into the cold start injector.  This air bypasses the throttle body and idle air supply so adjusting those does nothing. As soon as you get the new thermostat it should work much better.

[ja17]

The increase in RPMS during your split linkage test, by pressing the IP lever, indicates your engine is running rich not lean. Yes, and this could be caused by your defective WRD.

[Pag280]

By pressing the IP lever I am definitely adding fuel, therefore if the revs increase with the throttle butterfly closed it should be running lean as far as I know.
Please correct me if I'm wrong.

[Peter h]

Sure, the thermostat should work.
I think even if he admits air and doesn't run higher, he's too lean.
My opinion.
Peter

[ja17]

Sorry for the "brain fade". Pag280, you are correct that your system is running lean. A bad barometric compensator, among many other things, can also cause this condition. Make sure the WRD works before moving on to the baro.

[Pag280]

No problem JA17. I have already sent the order for new parts which should arrive probably later this week. I am going to replace the fuel tank and flexible hoses to start with clean 98 octane fuel. I have ordered the bits to overhaul the linkages, a valve cover gasket to check valve clearances, and a new thermostat for the WRD. I shall report after all the work is done. Thanks!

[col320ce]

You can tell when the wrd is no longer in warm up as there should be no (or only a tiny amount) of air still being sucked in through the filter. I find it easiest to test with the filter off and cover the opening with a finger. Also suck through the filter and make sure there is no resistance otherwise you'll need a new filter too.
Some have said on the forum that they need to drive the car to get it fully warm.

[Benz Dr.]

You can tell when the wrd is no longer in warm up as there should be no (or only a tiny amount) of air still being sucked in through the filter. I find it easiest to test with the filter off and cover the opening with a finger. Also suck through the filter and make sure there is no resistance otherwise you'll need a new filter too.
Some have said on the forum that they need to drive the car to get it fully warm.

You do. Any less and it may not be fully warmed up.

I would give the barometric compensator a half turn out to see if there's any improvement.

[Pag280]

So finally I managed to replace the parts that I had ordered including a new fuel tank and pipes, fuel filter, Throttle linkage overhaul kit and Warm Running Device Thermostat. After putting new fuel I tested the fuel pressure and the result was just under 0.8 bar.
I started the car to warm it up but it wouldn't idle, so I started again and held the throttle to keep it going. When the temperature got to normal I removed the filter from the WRD and no air was being sucked, so that is already a step forward. I got an assistant to hold the throttle for me while I could play with the idle screw but as soon as you let go of the throttle pedal the engine dies immediately. The only way I could get it to idle was to lengthen the FI linkage rod and set the idle screw 1 and 1/2 turns out. I went for a small drive in the neighborhood and the car was running fine up to 4k rpm, but if you increase revs it will not keep on pulling away, probably starving on fuel.
Any ideas how I can get it to idle on a properly adjusted linkage please?

[Pag280]

I also gave the Barometric compensator a half turn as Benz Dr suggested, but it did not improve.

[Benz Dr.]

I also gave the Barometric compensator a half turn as Benz Dr suggested, but it did not improve.

So give it some more until it starts to run better. 1/2 turn was only a guess.

[hansr433]

So finally I managed to replace the parts that I had ordered including a new fuel tank and pipes, fuel filter, Throttle linkage overhaul kit and Warm Running Device Thermostat.

I am having similar issues. Could you please list what was in your Throttle linkage overhaul kit with part numbers if possible and where did you buy it?

[Pag280]

In reply to hansr433 this is the linkage kit from SLS Germany

https://www.sls-hh-shop.de/main/en/mercedes-230-280sl-w113-/30-acc-linkage/set-small-components-acc-linkage-p-4410

[Pag280]

Today I started the car and let it warm up to normal temperature. Then I started fiddling with the barometric compensator, as Benz Dr suggested and after nearly 2 turns it started idling much better. So I went off for a test drive and the car was working better than last time, but after a few minutes it just died completely in the middle of the road. I managed to pull to the side and started cranking but it wouldn't start until I pressed the throttle pedal to the floor and it finally fired up. I drove straight to the garage and it died again in front of the door. I checked for spark just to make sure but it was ok. It is definitely looking like fuel starvation but fuel pressure is still around 0.75 bar and if I twitch the return pipe it goes up. I am honestly in the dark now, I know that I can remove the back cover of the pump and adjust fuel mixture but i want to make sure all the rest is good before i do it. Any suggestions please?

[mrfatboy]

What do the plugs look like?

[hansr433]

In reply to hansr433 this is the linkage kit from SLS Germany

https://www.sls-hh-shop.de/main/en/mercedes-230-280sl-w113-/30-acc-linkage/set-small-components-acc-linkage-p-4410

Thank you.  That's great.  I did not see that kit, even though I looked for it specifically.

With regards to your running issues, it sounds like a component in the fuel supply chain fails when it get hot.  You might have to eliminate/bypass one component at a time in order to find the culprit.  Have you checked that the air supply is as it should be?  Do you have a way to rig a temporary fuel supply pump/tank similarly to an engine test/break-in bench?

[Pag280]

What do the plugs look like?

Beige colour, yellowish. NGK BP6ES

[Pag280]

Thank you.  That's great.  I did not see that kit, even though I looked for it specifically.

With regards to your running issues, it sounds like a component in the fuel supply chain fails when it get hot.  You might have to eliminate/bypass one component at a time in order to find the culprit.  Have you checked that the air supply is as it should be?  Do you have a way to rig a temporary fuel supply pump/tank similarly to an engine test/break-in bench?

You're welcome!
I don't have another pump that would give the same amount of pressure.

[mrfatboy]

What do the plugs look like?

Beige colour, yellowish. NGK BP6ES

Can you post a picture?

[Benz Dr.]

Today I started the car and let it warm up to normal temperature. Then I started fiddling with the barometric compensator, as Benz Dr suggested and after nearly 2 turns it started idling much better. So I went off for a test drive and the car was working better than last time, but after a few minutes it just died completely in the middle of the road. I managed to pull to the side and started cranking but it wouldn't start until I pressed the throttle pedal to the floor and it finally fired up. I drove straight to the garage and it died again in front of the door. I checked for spark just to make sure but it was ok. It is definitely looking like fuel starvation but fuel pressure is still around 0.75 bar and if I twitch the return pipe it goes up. I am honestly in the dark now, I know that I can remove the back cover of the pump and adjust fuel mixture but i want to make sure all the rest is good before i do it. Any suggestions please?

I've seen this happen before. In most cases, the compensator was turned out too far and the plugs fouled out. Wherever it starts to run good I usually turn it back in a ways past that spot or this is what often happens. The compensator adjustment is a very touchy thing and you turned it way too far. Sounds like you could have a bad compensator if you had to turn it out that far.