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  #1  
Old 05-28-2018, 10:23 PM
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1971 W108 M180 dumb question about spark advance

When the engine is allowed to slow to idle when coming to a stop and the car is out of gear (mine is a manual) the idle speed drops after several seconds. Is that a change in advance because the manifold no longer creates enough force?
I am so confused by this basic functionality.
Thanks
Brad
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  #2  
Old 05-28-2018, 11:54 PM
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bc:

Try operating without any vacuum connected to the distributor (plug the line), and the idle timing set to ~10-12 deg. BTC.
If the delayed idle speed drop persists, look for a dashpot on the throttle linkage. If there is a dashpot, the delay is likely intentional. Part of the emission controls.

Last edited by Frank Reiner; 05-29-2018 at 08:44 AM.
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Old 06-02-2018, 11:26 AM
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Quote:
Originally Posted by Frank Reiner View Post
If there is a dashpot, the delay is likely intentional. Part of the emission controls.
There is a dashpot but it isn't effecting the mechanical throttle when the idle speed drops.
Let me describe what is happening in more detail. If I shift into neutral at say 40 mph and coast the idle speed stays at just under 1000 rpm. Lets say I take 20 seconds to roll to a stop under light braking the engine idle remains at 1000 After the car stops the idle speed remains at 1000 then after 5 to 10 seconds the idle speed drops to 600. The dashpot fully relaxes after just several seconds.
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Old 06-02-2018, 11:46 AM
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Most likely a slight carburetor imbalance which comes into play when the engine vacuum reaches maximum at sustained idle.

Try Frank's suggestion to eliminate advance first before diddling with the carburetors.
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Old 06-02-2018, 12:44 PM
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Originally Posted by Mike D View Post
Most likely a slight carburetor imbalance which comes into play when the engine vacuum reaches maximum at sustained idle.

Try Frank's suggestion to eliminate advance first before diddling with the carburetors.

Mike, if memory serves, the engine in question is equipped with Bosch mechanical injection. 280SE
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Old 06-02-2018, 01:30 PM
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Oops! Never mind. The SE part was never mentioned. That's what happens when you assume something.

I also didn't know the 1971 models still had the M180's. I learn something new every day.
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Old 06-02-2018, 02:30 PM
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Originally Posted by Mike D View Post
Oops! Never mind. The SE part was never mentioned. That's what happens when you assume something.

I also didn't know the 1971 models still had the M180's. I learn something new every day.

Likely that it is actually an M130; the last of the M180 line.
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Old 06-02-2018, 03:48 PM
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Originally Posted by Frank Reiner View Post
Likely that it is actually an M130; the last of the M180 line.
Oops. My bad I do have the m130. Is a 2.8 mechanical fuel injection.
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Old 06-03-2018, 03:21 AM
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I've seen where this was a distributor problem. Flyweights were sticky and would eventually close up after a prolonged idle. More common than you would expect.
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Old 06-03-2018, 01:35 PM
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Can I offer a possibility that is not common: The brake booster draws vacuum from the manifold. Is the any correlation between the brake pedal and the drop in rpm?
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  #11  
Old 06-03-2018, 07:19 PM
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Originally Posted by JeffreyNMemphis View Post
Can I offer a possibility that is not common: The brake booster draws vacuum from the manifold. Is the any correlation between the brake pedal and the drop in rpm?
There can be but that's usually only when applying the brakes. I would try pulling the vacuum line from the throttle valve to see if the engine changes speed which can tell you if the vacuum advance system is working.

Most of these old cars will need a distributor rebuild after 40 plus years of running.
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Old 06-06-2018, 12:10 PM
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Is the any correlation between the brake pedal and the drop in rpm?
No correlation. I know of the effect you are talking about but nothing happens when you pump brakes.
Brad
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  #13  
Old 06-06-2018, 12:15 PM
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BTW the engine is a M130 mechanical fuel injection

Quote:
Originally Posted by Benz Dr. View Post
There can be but that's usually only when applying the brakes. I would try pulling the vacuum line from the throttle valve to see if the engine changes speed which can tell you if the vacuum advance system is working.

Most of these old cars will need a distributor rebuild after 40 plus years of running.
The speed does indeed change so the advance it working. BTW we have had a cooler couple of days here in Nashville and I noticed that the rpm drop isn't happening with engine at operating temperature. So it seems to have something to do with when the engine is not only operating at 180 coolant temp but the rest of the engine is also hot.
Brad
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  #14  
Old 06-06-2018, 02:46 PM
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Check your fuel pressure and volume. You need at least 10 PSI ( 15 is better ) and one liter in 15 seconds to have enough volume and pressure. Test at the return line in the engine bay.
I also test the return line going to and through the fuel tank. Blow through with compressed air and listen for bubbling in the fuel tank. Remove the fuel cap and wrap a rag around your air wand before testing.
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  #15  
Old 06-07-2018, 08:52 AM
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Quote:
Originally Posted by Mike D View Post
Oops! Never mind. The SE part was never mentioned. That's what happens when you assume something.

I also didn't know the 1971 models still had the M180's. I learn something new every day.
I have been mistakenly identifying my engine as M180 when it is a M130 so you are right that M180s were not being installed in 1971.
Brad
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