re: 81 300D

Have any of you mechanics seen cases where a valve (or valves)take 'spells' not seating tightly/snugly during idle, causing miss, rough idle and puffing heavy smoke?

As I've posted before, my mb will idle perfectly one moment, and get rough the next. This is ONLY during idle, does fine when I accelerate. Also, it starts beutifully! cranks right off, no problem a'tall.

I talked to a local MB mechanic over weekend and he tells me it could be a valve not seating properly at times, thus causing these "spells" loosing compression and causing the smoke.

This would make sense, but why would a valve seat tightly one time on idle, and not the next? Seems to me it would be a consistant problem if the case. Thanks for any 2 cents, Phill