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.