What would cause the idle to be perfectly smooth at ~900rpm for 60 seconds, then suddenly bottom out around 500rpm and start bouncing? Then I tap the gas and it starts over

 

could be a few things but sounds like you need to change the fuel filter to start. ..when was your last tune up...start with the basic stuff ..may even want to try carb cleaner for the carb. Does the car run rough?? give more inof. and maybe someone else can pinpoint better for you....cjf

 

Sounds like it might be ignition related, how old are your plug wires, cap and rotor? that could be it.

 

My Cosmo POS kinda does that but its fuel injected, but anyways, check to make sure you havent got a dirty carb or something? Fuel delivery?

 

Thanks I will have to start thinking about the fuel and ignition systems. I'm pretty good about maintanence and tuneup; all the wires/cap/button/filters/plugs etc. have been changed within the last 6 months.

 

I just ordered a new anti afterburn line from mazdatrix with some other bits and pieces. The old one was split so I taped it. The car is actually running pretty damn good now since I got the mixture right. Taping the split line and fixing the mixture helped the idle alot, but it still hunts from time to time.

 

Its been my experience that hunting idles are usually from vacuum leaks. The best source for a vacuum leak that I've found so far is the shutter valve's tube that runs to the air cleaner. Block that off and see how it acts.

 

It's your shutter valve.

 

The shutter valve is disconnected. I have the smaller line open but the bigger tube is still running to the airbox.