I have a 78 L42 with a starting and idle problem also.  I bought the
 car with and Off-road exhaust and it ran very well but was very loud.
  I replace the off-road with a cat less muffler system and now the
  car idles very rough with stalling after it gets hot.  Cold idle is
  where it should be and seems to work correctly.  When hot the idle
  is between 500 and 550 which according to the engine sticker is
  correct for hot idle.  It jumps up to about 600 when in gear.  It's
  an automatic.  The air doesn't seem to change the idle.  I used
  carburetor cleaner and the idle seems to improve a little.  The car
  has about 38K.  I ran the car for a two hour trip and the car
  performed well although when I stopped for lights it seemed to idle
  very low but it didn't stall or stumble when I accelerated.  I parked
  the car in the driveway and ate lunch.  After lunch I tried to start
  the car again to put it away and the starter ran but the car didn't
  catch.  I tried about 10 times.  It would almost catch but not keep
  running.  I then tried with the gas fully depressed and was finally
  able to start the car.  With the cleaner off I checked out all the
  carburetor movements and all seemed normal except for the idle
  solenoid which doesn't seem to work correctly.  I'll try to get
  another one but I can't see why that would cause my problem.  Any
  suggestions?

Hey Leroy,

The exhaust change may, or may NOT have anything to do with the issue you are now having.
It sounds as if you may have a vacuum leak somewhere, but that usually creates a higher idle, not lower idle. However...you may have a problem with the EGR valve that could cause this problem. It's possible that when you changed the exhaust, it allowed enough backpressure reduction to let some crap blow out of the intake/heads, into the EGR valve, and hang it open a smidgeon(technical measurement). This can happen when you "nail" it after a modification, to see how it runs...

It can also be a carb problem. Once you did get the car started after holding the pedal down and cranking it, did it blow a lot of black smoke out the tailpipes? (did ya even notice?) That would definitely indicate a carb that may be leaking, or a float level that is too high, which is really what your description of how the engine is running points to...

The idle solenoid is what controls idle speed.  To check that there is no other problem, unplug the wire from the solenoid, and unscrew the idle stop of the solenoid until your car idles at a good speed.  If it now runs and drives well, you might be able to clean the solenoid if it just sticky, or else replace it.

Larry

I think mine is a Carter.  Any information on that one.

What Joel said, also if you can disconnect the exhaust and see if the problem still exsist, this will let you know if maybe there is a restriction in the exhaust.

The solenoid mounted on a 78 FACTORY carburator is just for the AC and has nothing to do with the idle adjustment(contrary to what the shop manual says).  Idle speed is adjusted with the screw on the side of the carb that contacts the linkage. In non AC mode, the soleniod plunger doesn't even contact the linkage. When the air is turned on the idle is simply kicked up about a hundred rpms to help with the added load from the AC compressor. mkapp78792008-10-23 20:04:56

Thanks.  I have a L42 and the engine tag says 5500 for idle and 6000 for car in drive.  My RPM's don't change with AC on.  I'll try with the idle selenoid out of the way and the idle at 5500 and see if the problem returns.

When you turn on your A/C the idle will balance out to regular idle (maybe a little higher) if the soleniod is working, you will be able to see it move by disconnecting the wire at it's back side.

[QUOTE=LeeW78]Thanks.  I have a L42 and the engine tag says 5500 for idle and 6000 for car in drive.  My RPM's don't change with AC on.  I'll try with the idle selenoid out of the way and the idle at 5500 and see if the problem returns.[/QUOTE]