Intermittent Miss

[Harry]

1966 230SL Automatic.  I just installed the Pertronix Election Ignition (mentioned in a prior post) with the associated Igniter Coil.  All seems to be running exceptional except I do recently developed a miss (that "feels" electronic) at idle.  I haven't delved into this yet but I thought I'd pose the issue to the Forum before going too far.  Also, the idle seems to wander a bit.  In gear it will often idle around 800 RPM - which feels right.  But sometimes it will drop to about 500-600, which feels low to me.  Out of gear, the idle may raise to 1400 RPM or so.  And anytime I apply the brake before putting into gear, the idle will raise.  So my normal practice is to apply the brake and wait just a few seconds to let the RPM drop to a level that might not be hard on the tranny.  (I recall the same behavior from these car back when my father had them.)

Any thoughts as I begin to seek this out?  And, I think the problem was there even prior to the Pertronix but it seems to have worsened recently (meaning that I am not convinced the Pertronix is a contributor).

Thanks,
Harry

[Raymond]

You probably have a vacuum leak in your brake booster or the vacuum lines from it.  Search the forum for Brake affects idle, or similar.  I know this has been discussed before.  I had that problem and solved it by having the booster rebuilt.  Can't recall the vendor in California who did the work but it's been several years without the issue returning.

[rwmastel]

And anytime I apply the brake before putting into gear, the idle will raise.

I second what Raymond said.  Back in 2000 when I got my car, I couldn't find a rebuilder, so I bought a new booster.  Wish I could have got my old one rebuilt, it had the original factory date stamp on it.

[Cees Klumper]

Additional causes could be a weak Constant Speed Solenoid, if your car is equipped with one, or the distributor may be worn or have some mechanical problem.

[Vander]

Here is a good way I like to diagnose if there is a brake booster/vacuum issue.

With your car idling and in park stare at your tach and press the brake pedal really HARD and to the floor, if the idle changes you have an internal issue with the booster.

[Harry]

Thanks to all!

[Pawel66]

My brake booster works fine and I also experience slight momentary rpm increase when I press the brake pedal - I think it is normal.

But back in the day when I had very rich idle - the rpm increases were significant upon pressing the brake pedal. And I had intermittent misfires.

I am not a mechanic, but before ripping the brake booster apart I would do a simple split linkage test to see if the idle is more or less set intact and if not, I would start from the linkage tour.   

[dirkbalter]

My brake booster works fine and I also experience slight momentary rpm increase when I press the brake pedal - I think it is normal.

Mine does that too. Rebuild booster.

[Pawel66]

Ok, sure.
I remember a discussion on this - with the conclusion as I wrote (cannot find it now), but if it is about booster rebuild - that is what it is.

[Vander]

I just installed a new booster in my 280SL today, and it fixed the symptoms on my car.