{Chrysler 300} WCFB unstable idle ... intermittent

[John Grady <jkg@xxxxxxxxxxxxxxxxx>]

In the interest of saving others a huge amount of hassle , after much time and effort , 5- 6 blind alleys , and much carb sleuthing , I found the “problem of the year  “ for me . Aggravating totally understates  it 

Big news , —-nothing was wrong with carb , no wonder could not find anything wrong . Worth listing the progression , as this info is new to me after 60 years of carbs. 
Symptoms : 
1) sometimes saw vacuum in distributor advance line , not supposed to be there . That adds to issue , possibly raises idle speed and impacts idle screw settings . 
2) Idle would randomly rise to 900 + rpm , dangerous , and when that happened carb would not be resting on hot idle screw tip , engine running . You shut off engine - it immediately moves and sits on tip ! WHY ? Choke stuff not in this picture at all. 
3 ) any messing with linkage when it is doing this especially accelerator pump link often made it act correctly (a red herring) 
4) # 3 led to noticing bowl vent cap ( lifts up off throttle) was hitting underside of air cleaner , possibly holding carb from returning ? Thought we had it . No 
5)    Adding a lot of spring ( way too much ) did not fix it (!!) , that one really baffled me. 


OK cut to it: 3 AM thinking —-

It turns out that the throttle shaft in all carbs is as loose as it is , for a good reason  , —the precision shaping and exact fit of throttle plate edge at idle port  is crucial to idle gas port flow , and to the transition slot to main jet working right , ported spark hole too . . and those plates “find themselves “ in the venturi bore , possibly requiring a slight AXIAL motion of shaft in and out along bore to center and finally seat (!) just as they close . 
So anything —that in any way —places a small axial force into carb shaft will tend to make them scrape on the side wall , just before they seat in bore  . 
****In this case the return spring was not exactly in a straight line with throttle rod due to a wrong or bent mount of the stationary end of spring ****
That caused a sideways pull on shaft sometimes preventing total closing - as it would scrape the wall at last minute  . Vacuum pull impacted all that , and when being held slightly open some vacuum gets into spark port which is like .030 over plate and that can raise idle and also messes up idle mixture , probably lean , which cause a pop back , or bog off idle . 
Much learned , keep all this in mind!! Keep those springs perfectly in line with the pedal rod!! 
Best ,
john g  


Sent from my iPhone not by choice 

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