A '78 would have the older-style idle valve. I am not sure where it is on the V8's - probably the top front of the engine. It is identifiable by the very large screw that is used to set the idle speed. It is mechanical and does not do very much. Try backing the screw out a bit to speed up the idle and see if that helps.

When does it stall? Hot or cold? Run OK otherwise? Hard to start hot or cold?

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe