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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? TIA
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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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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!
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Johan Limcangco Manila, Philippines 79 350SE (W116) 86 230E (W124) http://www.geocities.com/johan01.geo |
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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, Mark
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DrDKW |
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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, Mark
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DrDKW |
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