Idle up then down?

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TF360

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What do you guys think? 74 360- When I come to a stop in the car the idle is high, and then after a couple seconds it goes down to normal. Timing is good and it's not the linkage it is releasing. Also, swapped on another carb and it's doing the same thing. What do you guys think it sounds like?
 
Something that just started or been happening? What mods to engine? Larger cam needing more initial advance, being idled up can cause the vacuum to hold longer pulling advance to distributor. Or could be a vacuum leak causing the idle to rise enough to still allow carb timed vacuum to operate. Other thing I can think of is the advance plate in distributor is binding. Just some initial thoughts.
 
Dubob said:
Something that just started or been happening? What mods to engine? Larger cam needing more initial advance, being idled up can cause the vacuum to hold longer pulling advance to distributor. Or could be a vacuum leak causing the idle to rise enough to still allow carb timed vacuum to operate. Other thing I can think of is the advance plate in distributor is binding. Just some initial thoughts.
Click to expand...

vacuum leak........is my guess
 
Did not do it in the past. I tuned it up hoping that would help, and it did. I also turned the idle up a bit to keep it running when it goes back down.
 
You know my brake light is on, but when the booster is leaking vacuum it does not show on the dash light right?
 
Assuming a 4 bbl and moderate duration cam:
1. The secondary throttle stop may be open too far
2. The PCV may not be suited for the cam; we cured your exact same problem past week by changing PCV. The original Mopar item was open too much at the 11-12 inch idle from the moderate cam and letting in a lot of excess air. Here is the solution thread for a different PCV; it settled it all down very nicely.

http://www.forabodiesonly.com/mopar/showthread.php?t=335105
 
Either the fuel level is unstable or the transfers are feeding, cuz the primary throttle-blades are open too far,cuz the timing is insufficient.This same too-far open primary may be signalling the vacuum advance can, to pull in some timing. Then as the vacuum falls at idle, the vacuum advance drops out.
That's my guess.
 
Clamp off the rubber line going to the PB booster. See if it stops.
 
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