Help Understanding High Idle Mechanism 396

[Keith B.]

I need some help understanding the high idle mechanism on my 396. My car is always climate controlled started so I have really never thought much of the "cold start" for this car. It fires up so easy and idles steady wherever I set the RPM. I do get a slight RPM increase with oil warmup but not significant (500-1000). Once I start the car the motor holds heat very well so to do any testing takes almost a complete day before I would consider it cold. The motor is hot but the carb doesn't feel warm even after several minutes running. In addition the tube that provides warm air from the exhaust manifold also is not hot. How long would the car need to run before the idle would "kick" down. On the 3124 the butterfly is controlled by the choke where you can lean the carb (plastic adjuster). What actually creates the carb to set an idle that is higher at the start of a cold engine? There is no connection between the throttle and the butterfly. It must only be controlled by heat. If this is the case when started in what position would the butterfly be and where would it end up once warm. It stays in about a 45 degree position all the time. Does the butterfly actually make the idle higher? Really struggling with the concept.

Thanks

[Clem Z.]

Re: Help Understanding High Idle Mechanism 396

I need some help understanding the high idle mechanism on my 396. My car is always climate controlled started so I have really never thought much of the "cold start" for this car. It fires up so easy and idles steady wherever I set the RPM. I do get a slight RPM increase with oil warmup but not significant (500-1000). Once I start the car the motor holds heat very well so to do any testing takes almost a complete day before I would consider it cold. The motor is hot but the carb doesn't feel warm even after several minutes running. In addition the tube that provides warm air from the exhaust manifold also is not hot. How long would the car need to run before the idle would "kick" down. On the 3124 the butterfly is controlled by the choke where you can lean the carb (plastic adjuster). What actually creates the carb to set an idle that is higher at the start of a cold engine? There is no connection between the throttle and the butterfly. It must only be controlled by heat. If this is the case when started in what position would the butterfly be and where would it end up once warm. It stays in about a 45 degree position all the time. Does the butterfly actually make the idle higher? Really struggling with the concept.

Thanks

the spring inside the choke assy closes the choke butterfly and moves the fast idle cam so it make contact with the fast idle speed screw causing the base plate butterfly to be held slightly open for a faster idle. if your hot air tube to the carb does not get hot it is blocked or the vacuum that pulls the heat up the tube is not there. remove the tube from the choke housing,start the engine and see if you have vacuum at the choke housing where the tube attaches. if you have no vacuum the opening inside the choke housing,the passage in the carb body is plugged or the little seal between the choke housing and the carb body is missing. if you have vacuum then the problem must be in the line from the exhaust manifold or the exhaust manifold passage.

[Keith B.]

Re: Help Understanding High Idle Mechanism 396

When I move the choke assembly out of the factory setting it does close the butterfly but it changes the richnest of the carb. Should I take the three screws off and look at the spring? How does the spring determine idle speed or is it an adjustment made once the plastic adjuster is removed to actually get the idle bumped up. Do you tighten the screw or back it out?

[Clem Z.]

Re: Help Understanding High Idle Mechanism 396

When I move the choke assembly out of the factory setting it does close the butterfly but it changes the richnest of the carb. Should I take the three screws off and look at the spring? How does the spring determine idle speed or is it an adjustment made once the plastic adjuster is removed to actually get the idle bumped up. Do you tighten the screw or back it out?

the adjustment of the spring by turning the black bakelite part cause a change in the spring tension. the fast idle speed is controlled by which step on the plastic cam it comes to rest on and that speed is adjusted by a screw on the fast idle cam lever attached to the base plate butterfly shaft. you need a copy of the 1965 chevrolet interim shop manual # ST-71 which may be available from NCRS website