look at the schematic in your owners manual-
I believe your problem stems from the seat switch not being closed when occupied.
Being closed provides power to the Operator presence Relay #1.
IF OPR1 isn't being powered, the spark kill circuit is engaged.
Also, a bad connection from the seat switch to OPR1 could result in the same thing. (or even a bad relay)
IF the seat switch is working, disconnect OPR1 and see what happens.

The attachment switch provides 2 separate functions.
When OFF, it allows continuity to the Start circuit to the starter solenoid.
When engaged, it provides power to the carb fuel solenoid if the seat is unoccupied.

This post was edited by bill_kapaun on Mon, Jun 16, 14 at 0:47