This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
In cold weather, some vehicles with the 2.6 liter 4-cylinder engine may
have severe power loss (e.g. after running about 20 minutes between 60 and
65 mph). If one steps on the gas, black smoke may come out of the tailpipe.
The solution (from Dave Witte): A de-icing kit to heat the intake air
enough to prevent freezing. SOME engines had this kit at the factory. The
kit is not expensive.
My manual shows that the 2.6 had a carb air heater. The tube that comes up
from the exhaust manifold, will be on the back side of the engine, and hard
to reach. When the engine is cold, make sure that the vacuum actuator in
the air horn leading to the air cleaner housing is working. In cold
weather, with the engine cold, it should redirect all of the air flow
through the stove on the exhaust manifold. (Jeff Wieland)
The problem was freezing of the carb. and the answer was to moved the hose
that feeds outside air to the air filter compartment and position it
somehwere to the rear of the engine. This prevents the outside cold air
from making its way over to the carb.(Ken)