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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 |
#2
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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. |
#3
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Quote:
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. |
#4
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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.
__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.” ― Brian P. Cleary, You Oughta Know By Now |
#5
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Mike, if memory serves, the engine in question is equipped with Bosch mechanical injection. 280SE |
#6
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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.
__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.” ― Brian P. Cleary, You Oughta Know By Now |
#7
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Likely that it is actually an M130; the last of the M180 line. |
#8
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Oops. My bad I do have the m130. Is a 2.8 mechanical fuel injection.
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#9
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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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#10
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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?
__________________
1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o 1957 Ponton 220S 2001 S600 Daily Driver The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com |
#11
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Most of these old cars will need a distributor rebuild after 40 plus years of running. |
#12
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Quote:
Brad |
#13
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BTW the engine is a M130 mechanical fuel injection
Quote:
Brad |
#14
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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. |
#15
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Quote:
Brad |
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