Sudden high idle

[Jeremy911T]

I have a 71 911T that suddenly during a drive started idling aroud 1400 RPMs. I was stopped at a light and noticed the high idle. It eventually settled down to normal after about 10 seconds but now is consistently at 1400 while at idle. It had been sitting steady at 800 RPMs at idle. No other drivability issues. I realize I can adjust the idle down but I don't want to be chasing something and make a bunch of adjustments I will be chasing later. Car is pre-vacuum advance. Marelli distributer with Zenith carbs. Any thoughts would be greatly appreciate. Thanks, Jeremy

[EC900]

Take a peek behind the pedal cluster. There’s a pair of nylon bushings that fail (disintegrate) on the accelerator bell crank and can cause the throttle rod linkage to bind up on clutch linkage and will hold high idle until you blip the accelerator pedal to free it up.

[al lkosmal]

The mechanical advance could be sticking.
regards,
al

[Jeremy911T]

Thanks Al, It did turn out to be the advance. About a year ago I servied my Marelli distributor. I replaced the springs and lubed the felts and weights. Yesterday I took a look at the distributor and one end of one of the springs had come off of a post. This was not allowing the distributor weight to come back to center at lower RPM so it remained advanced. I pinced the spring loop a bit, put it back on and now it is purring at 800RPM at idle once more. Sort of unusual, but may be helpful for someone in the future chasing a similar problem.

[al lkosmal]

That is a happy ending!

[ahh911]

Have an 81 sc,

I'm starting to believe that though the distributor moves freely, the notch in the timing curve down low where advance meets delay makes rpm at 950rpm sort of jog around when the oil is warm, usually just above the trombone opening.
Maybe 800 to 850rpm set warm it will never move once warm, bug will be lower during warmup. When set to 900 rpm warm, lights on, clutch out, either or both will stablilize the rpm at 850/900, otherwise it might start bouncing off of 1050 rpm or so over a decently warm car, but unpredictable. So yeah, either my tach is off, which is possible or that inflection curve in the distributor is set wrong, or I'm wrong, I should get a light on there, I think I did a year ago and that was it.. Distributors make it fun (there's one I"ve never heard before)

Just looking at the graph and thinking about it, it should stabilize at the inflection point, maybe I should reduce the rpm and check that timing advances too? mechanical.
The slopes aren't the same, something else to think about.
Today was my last run, possibly of the season, 40k km, 25 M since the engine work, crazy, replaced tires, oil, valve gaskets, new pads in back, oil level gasket... hmm not bad indeed.
Maybe next year...

(didn't mean to thread steal.. but I thought applicable and interesting? and maybe wanted to get off my chest, like car therapy? The patient is always right )


Phil