[pbanders]

May be a vacuum leak, as Geoff says. I'd also see if you have a lean warm-up mixture. This may be due to a lower than required value of CHT sensor resistance during warm-up. First, eliminate all vacuum leaks. If the warm-up idle still oscillates, try putting a 50 ohm resistor (1/2 watt) in series with the head temperature sensor (splice in using spade connectors where the CHT sensor connects to the wiring harness). You can use up to 100 ohms of resistance without affecting the fully warmed-up mixture setting.