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'76 450SLC
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Discussion Starter #1
Well after giving up finding an accurate vacuum diagram for my '76 I took it to the shop. Three weeks and a couple paychecks later I got her back, running better.

There was a loss of power, especially in the low-end but it was driving. Strangely, now the RPMs in park are 1500 or higher, and it drops down to 750 in drive.

Curious as to how they solved my vacuum conundrum, I took a look. The current arrangement looks nothing like the Hayne's, and several valves are left without lines attached, including both nozzles on the left thermo-vacuum valve.

I adjusted the fuel/air mixture by the guide of lightly pressing the disc to where the RMPs stay consistent before dying, and the car finally had power, and drove great for a couple weeks. This even seemed to fix the delayed shift to second gear.

Now the car has started cutting off while driving, something I haven't experienced since replacing the spark plug wires six months ago. It often lurches forward and I have to let OFF the gas to get power sometimes. I had them check the fuel pressure, and they said it was OK.

In park I can't get higher than 3700 RPM, though I have read that some Benzes have this cutoff in Park and Neutral. Redline is 5700.

Also, the car started backfiring out of nowhere. I fixed that by adjusting the timing but I'm hesitant to do anything else at this point. Any ideas?
 

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1972 280CE
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I adjusted the fuel/air mixture by the guide of lightly pressing the disc to where the RMPs stay consistent before dying, and the car finally had power, and drove great for a couple weeks. This even seemed to fix the delayed shift to second gear.
Can you explain this a little better. There's usually only one adjustment and to richen and lean out the engine and that's on the air flow meter. It's not something that you should do by ear. But instead do it while using a CO meter.

Are you talking about the adjustment that requires a 3mm hex?
 

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'76 450SLC
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Discussion Starter #3
Yes, the 3mm hex adjustment. I had a scan from an old MB service manual, including instructions on how to adjust it by ear, in quarter-turns. I will see about a CO meter.

This minor adjustment seemed to restore the torque in first gear, but, it was only temporary, and besides, the jerks and "empty" acceleration remained. I have replaced both the fuel and transmission filters.

I will check and post the vacuum line arrangement soon.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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I admit I have played with adjustment but always mark the original position. A little at a time, wait, rev, wait. Return to original, repeat. Try other direction, repeat. Usually end up back at original. The exception is the 560. Accumulator was bad and I think it had been adjusted rich to compensate. Leaning it out made a difference after I replaced the accumulator.
 

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1976 450 slc
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everything I have read on the subject says the adjustment screw only adjusts idle mixture. It doesnt do anything to the feul / air mix throughout the rest of the rev range.
More info is needed- I have a 76 slc aussie delivered with no smog gear at all.
If yours has a thermo valve it had an EGR valve, does it also have a air pump, if so what is still connected?
There are other things at play here including the way your vac lines are currently routed, condition of your rotor & cap, T/C stretch, AAV & vac advance on the distributor working correctly just to name a few
 

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'76 450SLC
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Discussion Starter #7
If yours has a thermo valve it had an EGR valve, does it also have a air pump, if so what is still connected?
Air pump is connected and a bit erratic. It regularly pumps warm air into the cockpit even when in the "OFF" position in the dash. The vacuum line which I installed from the diverter valve to the black thermo-vacuum valve has been removed.

Another thing is that it often takes an extended crank to turn over. This is shortened if the car has been running previously that day.

I have attached the original Haynes diagram as well as my current setup. The warming up governor is attached to the contour hose, which is part of the ancillary fuel diagram, along with the pressure damper.

I took a look at the rotor a while back and it looked fine, but will replace it anyway, just in case.
 

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1972 280CE
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A common way to cover up running issues on these cars is by dialing up the richness in order to mask the true issue.

As I said earlier, quarter of a turn is considerable. A 5 degree turn is enough to change the CO level from 1 to 3%.

You have to find out what the core issue and tackle it with a methodical approach.

Check the following:

Vacuum system for leaks
Ignition system
Timing
Fuel delivery

The extended crank sounds like a fuel accumulator. Common symptoms with this is that the car will crank better when cold then warm. Unless you crank the car within a few minutes of shutting it off.

You need to make your best efforts to bring everything back to spec before fiddling or otherwise, you'll just end up chasing your tail.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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Air pump is connected and a bit erratic. It regularly pumps warm air into the cockpit even when in the "OFF" position in the dash. The vacuum line which I installed from the diverter valve to the black thermo-vacuum valve has been removed.
That is not the A.I.R. pump. He was referring to a smog pump on the front of the engine driven by a belt.
 

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'76 450SLC
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Discussion Starter #10
Yes, the belt to the emissions pump has been removed. Perhaps this negates the diverter valve, but I wasn't sure if the vacuum system should be closed.

I understand that a number of 107 owners have bypassed the emissions system and have open valves.

I ran new lines and hose for the entire vacuum system, and the workshop tested for leaks.

Since in Texas there is no emissions check on '76 models, I will probably have the cats removed. Will this affect CO readings for nailing the fuel/air mixture?

Thanks so much, guys.
 

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1972 280CE
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Removing the cats will allow you to measure CO from the tail pipe. This cannot be done with cats in place as they convert CO to CO2.

However, I'm pretty sure that on a 76, the cats are part of the manifold and therefore you'll need to replace that too.
 

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1976 450 slc
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looking at the vacuum diagram you posted the vac system to the air ( smog ) pump is removed completely.
Still the circuit to the EGR seems partly intact.
Your green line (2) runs through a vacuum swith before providing advance to the distributor.
On my car the connection 2 runs straight to the distributor with nothing in between, that is straight from the throttle body to the advance side of the vac element on the distributor.
PersonallyI would remove all associated stuff with the EGR valve and cap of any vacuum connections if it is not in use but first I would perform the test below
Test to see whether you are getting vac advance in your current setup, use a timing light to see if you are getting the required amount of advance at 2000 rpm from memory @ 20 odd degrees ( refer to manual)
If not remove dist cap and vac line from advance side and suck the port to see if the table is moving inside- if it is, there is a problem with current routing of vac lines, if it dosent move the table inside the dist is gummed up or the diaphram inside the advance pod is stuffed.
The above test is easy and will help eliminate a potential cause of your problem
 

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'76 450SLC
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Discussion Starter #13
Thanks for checking your setup, and for the suggestions. I will perform the test this week and post what I find.
 

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Outstanding Contributor , Bob's Your Uncle!
-----'83 280 SL----- 5 speed....The PIG
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1976 450SL, 1945 Willy's CJ2A, 2007 Dodge Nitro
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I'd like to see how this went. My air pump is bypassed as well as my egr valve. I'm pretty sure my setup isn't right. The car runs ok but I'd like to see a decent diagram of a 76' working properly with smog bypassed.
 

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'76 450SLC
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Discussion Starter #17
So I bypassed the EGR valve and plugged all vacuum connections, no leaks. I replaced the distributor rotor just to be safe.

I was set to check the timing, but the engine, after a rough and inconsistent idle, died every time either before I could adjust the distributor or immediately following the slightest rotation. If I gave it any gas it died on the spot. At one point, the disc of the air assembly was vibrating up and down like mad but adjusting the mixture returned it to idle.

Now, the car refuses to turn over at all. I sprayed some starter fluid into the assembly and she turns over, only to die immediately after. I can't help but feel it's an issue with the mixture but I've done minor adjustments for hours without luck.

I hear the fuel returning to the tank and swooshing around after turning over, and I loosened the metal line going from the distributor to the pressure damper just to make sure fuel was being delivered. The guys at the shop said pressure was ok when they checked it a few months ago, but who knows.

Any ideas?
 

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Discussion Starter #19
Enlisted the aid of a friend to start the engine while I pressed the plate following the suggestions here.

I had to retard the timing by 20 degrees, and now the distributor is turned to the very bottom of the guide-- it had been set in the middle when running fine. But now the engine runs very low in a series of two rough pulses, then eventually dies.

Adjusting the mixture in either direction seems to make no difference, and I can press the air plate all the way down without affecting the engine-- just a loud breathing sound. A whisp of light smoke sometimes pushes out when the engine dies and the plate is open.
 
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