Satisfy my curiosity

[rumrumm]

I  have always found this to be a weird little quirk with my car. I have a 383 in my  '32 that runs great. But the quirk involves idle. I have an Edelbrock 750 atop an Edelbrock Performer RPM intake, and it idles nicely at a little over 800 rpm. But once I have driven it ten miles or so, the idle jumps up to about 950 rpm. Is it heat from the engine compartment that causes this or what? It's not a big deal, but I would like to know why.

[kb426]

Lynn, my educated guess is that the warming up of the engine and the heat under the hood. Lean idles are always higher than rich idles. If the heat causes the carb to lean out some, this theory would be correct. My exp. is with blown fuel and alky so I might be the last person you would want to listen to.

[wayne petty]

I  have always found this to be a weird little quirk with my car. I have a 383 in my  '32 that runs great. But the quirk involves idle. I have an Edelbrock 750 atop an Edelbrock Performer RPM intake, and it idles nicely at a little over 800 rpm. But once I have driven it ten miles or so, the idle jumps up to about 950 rpm. Is it heat from the engine compartment that causes this or what? It's not a big deal, but I would like to know why.

i put up several links that you might want to read on this thread on tuning the afb/avs  

http://www.roddingroundtable.com/forums/viewtopic.php?t=13018

depending on how lumpy the cam is.. you really should be idling at a lower speed.... and only on the idle ports.. not on the idle transition passages...

the idle circuit is at about 8 minutes in.. or page 4 in the PDF below

http://www.mymopar.com/downloads/mtsc/273.pdf

these are the basics of how it operates...

most of the current issues are the primary throttle blades are open too far exposing the idle transfer slots..   these have a fixed idle feed restriction that is behind the idle air bypass bleed in the side of the booster..

people have drilled the primary throttle blades to allow additional bypass air to allow the throttle to close far enough to regain idle mixture control... when there are several other ways to perform this.. having a proper sized PCV system.. or opening the secondary throttle blades a tiny amount..  some holley and other versions are starting to have the hidden idle air bypass screw under the air cleaner stud again..  since once you have set the idle speed .. you really don't need to mess with it...

i don't know if your thunder carb is equipped with something that carters were.. back above the secondary venturi was the hot idle compensator.. its a layered device held down by 2 screws with a strip of metal and a button in the end.. the metal is  a bimetallic strip.. it bends when the carb gets hot and allows the button to uncover an additional air feed.. this raises the idle speed when the carb gets hot... perhaps thats whats going on and it would be normal...

a proper sized PCV will drop the idle speed between 50 and 80 RPM when you block its flow either by plugging the end or pinching the hose off..

if the PCV calibration is too big.. it will reduce the idle speed a greater amount and throw off the calibrations designed into the carb for it..

if the PCV flow calibration is too small.. and it drops less than 50 RPMs.. you will have the throttle open too far to compensate and expose too much of the idle transition slots..

sorry to bombard you with all this... it took me a LOT of years to put this all together...



knowledge is power...

and since we are not born knowing how to tune an afb...

the RRT was born..

Wayne...

[rumrumm]

Thanks, Wayne. This is just the kind of information I was looking for.

[Mikej]

Carbs should be set at operating temp.