280C Rough Idle When Cold

[ctaylor738]

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

[johan01]

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!

[Mark DiSilvestro]

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

[Mark DiSilvestro]

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