Jeff - it has to do w/ the dew point - certain humidity at certain temps - I fixed a 250 I had that insisted on carb icing under conditions that didn't affect other's in the fleet - scrounged a heat shroud from an exhaust manifold at the wreckers and put a tube from it to the air cleaner - the slight warming of the intake air cured it immediately

a way to deal w/ it is to start the engine, idle for 2 minutes, shut it off, wait 2-3 minutes and start it again, will usually have spread enuff heat to be ok

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics