The diode referred to in the service manual is the two that power the anti after-fire carburetor solenoid. They also feed the DSAI coils if present. Note these are no longer available and must be upgraded to the standard CDI Fixed coils. Easy enough to find if they are good as the carburetor solenoid will click when they are good.
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But what being described here is the failure of the internal steering diode in the ignition coils. Can be one or both coils has the diode shorted. Usually both coils has to be replaced or you can try installing a Briggs style kill harness with external diodes. This something I have yet to try out but in theory should work. Should be able to use a pair of 1N5408-G diodes. Connected so each coil does not backfeed the other coil. The anodes should connected to each coil with the cathodes connected together so when they are grounded the ignition coils are killed.