Fluctuations in idle on a tii are usually caused by the idle mixture being too lean, but I'll agree that 750 to 4500 rpm is a BIG fluctuation. Ignoring the size of the fluctuation for a moment, idle mixture is adjusted by the tiny screw inside the "tuna can" at the top of the throttle body. Backing the screw out (turning it counter-clockwise) makes the mixture richer. Tiny adjustments should be made. On every tii I've ever owned, which is something like 12 of them, you find the point where the idle no longer hunts, and where, when you blip the throttle, it doesn't drop down low before recovering. The fact that you say the engine runs well at startup and that the problem begins once it's warmed up makes me suspect something in the warm-up regulator. Its job is to give the engine extra air at cold start and during warm-up (and to move the enrichment lever on the Kugelfischer pump, which gives it more fuel). Once warmed up, the piston in the warm-up regulator should rise and move the enrichment lever so the screw on the enrichment lever (the "verboten screw") is resting on its stop. If it's not curable via mixture adjustment, and if the warm-up regulator appears to be working properly, the problem may be a vacuum leak. A smoke machine test may be helpful, as it may uncover a bad intake manifold gasket. But be aware that there is some amount of normal air leakage through the warm-up regulator, even when the piston is fully extended. Have you or has a garage put an exhaust gas analyzer into the tailpipe to see how rich or lean the mixture is when the idle fluctuates?
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