Erratic Idle

[Harry]

1966 230SL, automatic.

Hello again.  My 230SL has developed a behavior at idle and in gear that is a bit puzzling.  I can come to a stop and it will sit there for a while and idle nicely but then it will begin to stutter/stumble.  I can pump the gas a little and hold a little accelerator to it and it will run perfectly.  If I take it out of gear while stopped, the idle will increase to maybe 1500.  When driving down the road, it runs like a top.

Some perhaps relevant background is that I just replaced the points with Pertronix electronic ignition.  The capacitor is totally disconnected and the ballast is also no longer "in circuit".

So this afternoon, I checked the timing and while good at idle (although the idle is fluctuating a bit), it would not advance to 30 deg BTDC.  It would only go to 20 deg.

Honestly, I don't think these two anomalies are related but I thought I'd ask the Forum for thoughts/input.  Whether related or not, I need to find the issue with the advance.

And - it is a BOOGER to see those timing marks eve, with them highlighted in white.  I can see them but it is a very small aperture to look through.

Thanks,
Harry

[rwmastel]

I can come to a stop and it will sit there for a while and idle nicely but then it will begin to stutter/stumble.  I can pump the gas a little and hold a little accelerator to it and it will run perfectly.  If I take it out of gear while stopped, the idle will increase to maybe 1500.  When driving down the road, it runs like a top.

Harry,

In another recent thread you mentioned the idle going UP when you have your foot on the brake and we suggested getting the brake booster rebuilt/replaced.  Now you're saying the idle is stumbling when at a stop with foot on brake.  Have you made any adjustments to fuel/air/spark in the last couple weeks?

[Harry]

I do have the idle increase going up somewhat, i.e. it definitely is affected by depressing the brake, but based on the responses to that post and my experience when my father had these cars, I was convinced that is not necessarily abnormal.  Also, I can't imagine that it is tied to the erratic idle.  So I have proceeded accordingly.

In pursuit of the idle issue, I thought I should first be certain that car is properly timed.  That's when I found the issue with not fully advancing.  And I'm not convinced that even that is a contributor but certainly something that needs to be addressed.  All plug wires are tight.

It's always a science project for me to determine whether a miss is ignition related or fuel related but something has me thinking this may be a rich condition at idle since pumping the accelerator just a bit clears it up.  Of course, it might come right back but if I hold a little accelerator to bring the idle up a bit, the problem isn't there.  It could also be fuel starvation but it doesn't "feel" that way.  May be totally wrong.  Just curious whether there ahs been another member of the forum who has experienced this exact problem.

Thanks again,
Harry

[rwmastel]

I do have the idle increase going up somewhat, i.e. it definitely is affected by depressing the brake...

... but something has me thinking this may be a rich condition at idle since pumping the accelerator just a bit clears it up.

Could be, because if you're a bit rich at idle, and you press the brake and air leaks into the intake, the idle would go up.  I think people would agree your booster (or the vacuum line or check valve) needs repaired or replaced.  Until then, try the split linkage process to get that proper mixture at idle, and examine all the linkage connections for wear/slop.

I wonder if the inability to advance the ignition is also related to a vacuum booster problem.  I don't know anything about Pertronux ignition.  Does it still depend on vacuum?

[Harry]

Good advice .  Thanks.

[rwmastel]

Until then, try the split linkage process to get that proper mixture at idle, ...

And, I just thought, try the same process with a person (or a device) pressing firmly on the brake pedal.  You will see a difference in adjustment if the brake booster is allowing air past and leaning your mixture.

[rwmastel]

Some perhaps relevant background is that I just replaced the points with Pertronix electronic ignition.  The capacitor is totally disconnected and the ballast is also no longer "in circuit".

So this afternoon, I checked the timing and while good at idle (although the idle is fluctuating a bit), it would not advance to 30 deg BTDC.  It would only go to 20 deg.

Also, still thinking on this one, perhaps with the pertronix ignition parts install, your distributor got rotated by accident in the process?

[mmizesko]

Did you mean Eric Idle?  Very Funny. I'm Hilarious.

Mike

[Harry]

Eric Idle.... 

[Harry]

Can a replacement brake booster be sourced anymore?  And I'm not strictly interested in originality.

[BobH]

Hello, most of the usual vendors supply these, but you may be better off getting yours rebuilt, certainly less expensive, there are a few rebuilders mentioned on here

https://www.sl113.org/forums/index.php?topic=32294.msg234622#msg234622