I am NOT a guru, but have had a little experience with my daughter's
1980 245. One problem that we encountered that took a LONG time to
evaluate/correct was vacuum leakage in the auxiliary air valve hose.
It fits on fairly snugly but it took a hose clamp to really solve a
problem like you describe. Other vacuum leaks could do the same, so
you would do well to examine the whole rig closely, possibly including the
manifold gasket, for vacuum leaks. Cracked hoses and loose-fitting hoses
are prime candidates for problems. The control pressure or "warmup" regulator
is also an item that could contribute to that problem.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma