Intermittent Temporary Rich Engine Start-Up Condition?

[Paul462]

T337C N2576S Intermittent Idle Anomaly Description

Upon startup, the rear engine (Continental TSIO-360) sometimes runs rich, as evidenced by black smoke emitting from the exhaust. The throttle becomes very sensitive – the amount of throttle travel which would ordinarily yield an RPM increase from 1200 RPM to 1300 RPM actually elevates the engine speed to 1600 RPM; pulling the throttle back a travel amount which would normally reduce RPM 100 or 200 RPM then reduces engine speed to around 800 RPM. Bringing back the mixture control around two inches produces normal throttle operation.

The problem goes away before or during run-up. The engine runs normally during the rest of the flight. Normal and lean-of-peak magneto checks are normal. That would appear to leave either excess fuel or an air restriction causing this intermittent rich condition upon startup and for a few minutes afterwards.

We could run the fuel calibration process with the fuel flow gage, but that would only be effective when the intermittency is not occurring, which wouldn’t solve the problem when the intermittency occurs.

Any thoughts as to what could cause intermittent, temporary, abnormally rich operation for a few minutes following rear engine start-up?