Al, have you checked the auxiliary air valve? I checked mine by removing from the engine and heating it up on a pot of water in the kitchen to check that it was in fact moving (wife was not too amused!). Works basically like a thermostat - as the engine heats it up, the valve closes and less air passes thru it. I'm no expert, but maybe something to check...

Trevor

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2006 C180K 112000km
1980 450SLC 123000km
and a Toyota