Leaning out at Idle
So my car runs great from a performance standpoint. However when I sit for long periods of time then it slowly leans out. Last night I was stopped at a railroad crossing and when I stop I'm at 13 to 13.5 AFR at 180 degrees engine temp. Then it slowly starts to creep up to 14 AFR and my engine temp climbs as well. When it starts pulsing between 14.5-15 AFR the engine starts stumbling and running rough like it is missing but does not die. The AFR can get erratic at this point. The engine temp stays at 195 which is the thermostat opening temperature. I have the same problem in the summer but it tends to happen sooner. When I was sitting at the crossing last night I was idling for nearly ten minutes but only started stumbling for the last minute or two. I'm stumped, any idea what is wrong?
69 Winter Blue 302 Fastback w/ Original Paint