Help with vague return to idle

[cleandan]

I am having trouble with a carb that has a vague return to idle.
Here are the details so far.
Autolite 4100 (1.12 if it matters) in excellent rebuilt condition.
I can adjust the idle with precision both up and down in rpm.
There is no binding with any portion of the carb linkage. (each portion has been isolated in an attempt to find a fault)
There is no binding with any portion of the cars throttle linkage. (each portion has been isolated in an attempt to find a fault)
There are two good return springs.

I have verified the butterflies are not touching anything during their sweep from full open to full closed, including the gasket or the intake runner.
I have verified nothing on the carb is loose or bent (throttle lever rivets, shafts, butterflies on the shaft, choke step up cams and so on)
The throttle screw, and its spring, are in good order.
I have verified no part of the throttle linkage is having any identifiable issues.

Everything about this carb, and its set up, appears to be in order yet here I am with a weird idle that seems to alter itself at will.
The idle is a bit finiky when cold, but when warmed up a little things are good (this is more choke than anything else I suspect, as in I don't use the choke at start up, I just baby the throttle for the first few seconds until things settle in)

But when driving around I may pull up to a stop sign and get my "set" idle, or I may get an idle that is a few hundred rpm higher.
The odd range is in the 700-1200 range and never above that.
Sometimes it settles into the set idle slowly too.

I have even checked the system for worn spots that may have a ridge or groove in any part of the linkage that is hanging up, or acting like a variable cam, but nothing like that was found.

I have dealt with many different reasons why a throttle may not return to the same position, but in this particular situation there is nothing that shows itself as the culprit.
Something is there, I just can't seem to see it at this time.

Any good suggestions about what to check.

Oh, this is a rod and lever throttle linkage, no cables.

Thanks in advance.

[cleandan]

At the moment I posted I had a thought about what might be happening.
I wonder if I am losing my engine/or trans, mounts?
If they are shifting just a bit, but not yet failed, this might cause the engine to sit in a different spot after any hard romps.

Because this set up is using a rod and lever throttle linkage that may be just enough movement to change the idle slightly.

[GerryP]

Your ignition advance mechanisms -vacuum and centrifugal- are sticking.  As long as you aren't sitting with the engine at idle and it doesn't start climbing.  The advance mechanisms would only affect the engine returning to a low idle.  If it's not that, then the engine is getting unmetered air or fuel.

[6667fan]

When it is in the 700-1200 range is it fluctuating or could it be any of those rpms in that range that it settles on?

[shady]

What I would do is the next time you pull up to a stop sign and it is idling fast, I would lift the hood and start wiggling at the carb linkage and secondary linkage and maybe give the vac adv. and pcv valve a sharp rap with the blunt end of a screwdriver to see what happens.

[fe468stroker]

Also try to lift the gas pedal from underneath with your foot.  If it returns to set idle something is in a bind from pedal to carb.  Another test would be to quickly stab at the pedal and release seeing if it returns to your setting.  Good luck.

[1964Fastback]

The distributor springs being too weak was the first thing I thought of.  Although I did have an identical problem that turned out to be choke related.  On mine (Holley 2 barrel, part of 6v setup), the choke was almost but not 100% fully open.  Sometimes the fast idle screw would just barely catch on the lowest step of the cam, bumping the idle speed.

Pat

[cjshaker]

Your ignition advance mechanisms -vacuum and centrifugal- are sticking.  As long as you aren't sitting with the engine at idle and it doesn't start climbing.  The advance mechanisms would only affect the engine returning to a low idle.  If it's not that, then the engine is getting unmetered air or fuel.

My money is on this^^

It can even happen at idle if the springs are too weak to hold the advance weights. Especially if the idle is a bit above what is stock due to a bigger cam. Been there, done that.

[machoneman]

If you still suspect the motor mounts, brake torque it with the hood up. Easier with an auto trans but still doable. Just be sure the car is outside!

Saw quite few cars long ago at our Standard Oil station with broken, worn out driver's side mounts and a few with BOTH totally shot.

[cleandan]

While the motor mounts are sagged and in need of replacement, they are not broken so the engine is still under normal movement.

I did find the light advance weight spring floating a bit, meaning it was slack rather than under a tiny bit of tension when the weights were at full return.
I bent the post just until the spring had a slight amount of tension when the weights were at full return.
All parts of the advance system are working smoothly though.
I have not yet driven the car so I don't know if this worked, but it is a good idea and I thank those who presented it to me.