Well both too rich and too lean mixture can cause part throttle problem, but when the
mixture is too lean that usually causes backfire into the intake manifold, and the engine
responding better at the moment of pressing the accelerator pedal.
There could be an other reason of tumbling: defective return spring in the distributor's
vacuum chamber. If you set the basic ignition advance with this defective return spring,
the advance will be higher at part throttle. But this is explains only the tumbling, not the
change in the driveability after 30 min.
Last edited by LoneWolf33; 04-11-10 at 09:13.