High to Low Idle Speed

[FrictionTape]

Wonder if anyone has experienced their idle going to almost 2k then dropping to under 1k and almost stalling. 
 

Here’s a video. Thoughts?

 

 

[John76]

Looks and sounds like the idle is smooth, so probably not a carb issue or a vacuum leak. However, check your fuel pump and  filter. Fuel lines should be good, and no gas fume smell in the trunk or cabin.

Perhaps the distributor advance/retard needs servicing (clean, lube, check springs, vacuum diaphragm).

What kind of ignition? Points/condenser or electronic?

Only three things that change the idle speed:  Fuel, Air and Ignition.

John 

[TobyB]

Yep, I've had bistable idle before- turned out to be the centrifugal advance in the base of the distributor.

 

Someone had been in there, and both springs were light.  When one failed, the other wouldn't 

drop back to baseline until revs dropped below about 1400- which they wouldn't do with the timing advanced too far.

 

Timing light figured it right out.

 

t

 

[FrictionTape]

This is great insight, thank you!

['76mintgrün'02]

It's odd that it drops and then climbs and drops again.

 

The early style distributors have delicate little springs that rub on the underside of the center post's lobes and wear thin.  They get even weaker as that happens.  They slide on a thin (phenolic?) pad that wears through and breaks into pieces that can get in the way of the weights, plus they can get sticky without that pad.  The points plates in vacuum style distributors can get sloppy too, affecting the dwell, which affects the timing.  There are a lot of ways for problems to develop inside an old distributor.  As Toby suggested, a timing light will tell you whether you need to dig into it.  You can learn a little bit from a basic strobe style light, but a variable advance light will tell (teach) you a LOT more.

 

The AFR gauge that Toby talked me into was money well spent.  I look over at mine all the time while driving.  I'm glad it's mounted up high on the dash.

 

I also enjoy the vacuum gauge that's T-ed into the line feeding the distributor (manifold vacuum).  I have that one lower in the console.

 

You might want to inspect the throttle linkage too.  Which carb are you using?  Which model distributor?