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Old 02-05-2002, 08:49 AM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
280C Rough Idle When Cold

I am just not getting anywhere with the rough-idle-when-cold and related problems on my 1973 280C. Here are the facts:

- rebuilt SB and head, approx 500 miles
- valves adjusted
- points, plugs, wires new
- old plugs looked OK
- dwell steady
- timing at ~12 BTDC with vacuum at idle, appears to have normal advance as RPM's increase
- 4A1 carb rebuilt by good shop by PO
- carb adusted (idle mix, choke gap and fast idle speed)
- all smog lines connected
- EGR valve removed and manifold connection plugged
- secondaries work
- original CD ignition
- can't find a vacuum leak
- thermostat working

Here are the symptoms:

- starts instantly
- idles very rough, as in will not run below 2K RPM after starting.
- misses at low RPM driving when cold
- gradually gets better as warms up but not smooth until past 175 on the guage
- slight backfire on light acceleration when cold
- slightly rough idle even when warm
- slight hesitation/miss under light load even when warm
- slight surging at low load - like coasting to a stop - when warm

I have exhausted my knowledge and diagnostic equipment. On different days I think it is the carb, a vacuum leak, or ignition or all three.

The techs in the area are all over the place in terms of where they would start with it

Ideas, suggestions?


Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 02-05-2002, 10:54 PM
Registered User
Join Date: May 2001
Location: Manila, Philippines
Posts: 66
Hi Chuck,

Your description of your problems sounds much like my 250T and 280S before.

Have you checked the coil? There's a spec for the resistance between the terminals -- sorry, I don't know the right spec, but there is one. What about the condenser?

I assume you have the correct solid-core plug wires with the resistor boots and non-resistor non-platinum plugs?

Now about that carb.... could it have internal vacuum leaks? Could it have warped after the PO had it rebuilt? (ie: over-tightened when re-installing)

FWIW, my driveability problems with the 250T went away when I changed the Solex 4A1 to a Weber 40 DCNF.

However, on my 280S, it was all ignition... and setting the points and timing correctly fixed it. Now I have PerTronix kit.

Hope this helps, or at least hope it leads to another idea that would help...

good luck and have fun!
Johan Limcangco
Manila, Philippines
79 350SE (W116)
86 230E (W124)
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Old 02-06-2002, 02:50 PM
Registered User
Join Date: Jan 2001
Location: Alexandria, Virginia
Posts: 5,480
How do the plugs look? Black - gas fouled?
When Idling cold, does the idle improve if you open the choke slightly - too rich, maybe choke pull-off malfunction?
Does the idle improve if you choke the carb more or hold a rag over the throat - too lean? Does stepping on the brake make a difference - possible vacuum leak from the brake booster. Hope this helps.

Happy Motoring,
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Old 02-06-2002, 03:03 PM
Registered User
Join Date: Jan 2001
Location: Alexandria, Virginia
Posts: 5,480
I just remembered, on that Solex 4V, the secondary air plates (look like choke plates) must work properly or they can cause driability problems. As I recall there's a diaphragm, like a choke pull-off, that regulates the opening of these plates. The plates shouldn't open untill when or after the secondary throttles open.

Happy Motoring,

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