H8D-67

I have noticed for a while that after doing some hard driving or extended freeway driving that when returning to idle my olds will run about 250-300rpm higher than normal. I have dealt with it for months and just not worried about it because the car runs well but now I feel like figuring out whats going on.
Its a 425 with aluminum heads, performer intake with pro systems carb, MSD 6AL ignition. After hard driving it will keep that higher idle around 900-1000rpm for maybe 10 minutes of normal street driving afterwards then calm back down to normal. Anybody have an idea what it might be?

classicmuscle442

Throttle not returning to idle when released? Return spring weak or more likely throttle plates hanging up in carb bores.

coppercutlass

Advance weights could be hanging up. have you revved it after to try to lower the idle ?

H8D-67

Timing is locked out currently. I have tried blipping the throttle and it doesnt seem to change it at all.

H8D-67

Ive changed return springs and the issue has stayed. Could be throttle plates hanging up but I would hope not. This is a customer built pro-systems carb and isn't very old.

stlbluesbrother

If you shut the engine off and then restart, does it idle at the proper RPMs? If so, coppercutlass has likely explanation as it relates to advance weights in the distributor not returning to position.
Update: just noticed the comment on "Return springs ...".

H8D-67

Alright so took out the olds today. Did a quick hit and it started idling high. Pulled over and ripped off the air cleaner, throttle was sitting at idle like it should. Blipped it a couple times underhood and nothing changed. Turning the car off for a minute or so did bring it back to normal. Ignition setup is a msd 6al box and billet distributor with timing locked out at 32 degrees. So there is no advance weights to get stuck. Running a slightly colder plug for the 100 shot of nitrous that it gets hit with every so often. This problem did exist with previous hei setup as well.

74sprint

Some quadrajets and other carbs had a HIC or hot idle compensator circuit which would raise the idle when the engine got a little warm. Here is one site http://442restoration.homestead.com/carbs.html But I suggest doing a Google search for Hot Idle Compensator and see if this might be relevant to your situation.

Ray

classicmuscle442

Still think your problem is carb related. Without a computer the carb is the only component that can change idle speed.

shiftbyear

Is there a way you can get a timing light on it after the idle rpm's increase, just to rule out ignition. Also when the idle is high, see if the secondary throttle plates can be shut further with your fingers, by pushing the linkage arm. Some times the secondaries can hang up in their bore even thought the primaries are against the idle stop. Also check for vacuum leaks. Good luck.