The seat or reverse switch is not working properly so it shuts down the engine by killing the magneto
Because it only kills the magneto the sarb solenoid remains open so you get an after fire through the exhaust
If you have the 4 wire flat plug on the seat switch undo it spray some contact cleaner in it then pulg & unplug it a few times.
If you have a compressor blow the connectors out from the wire side to blow the crud out.
MAke sure that the plug is very tight as it has a shorting strip down the bottom so you can noy unplug the seat switch like 50,000 idiots on Face book / you tube tell fools to do .
Follow the wire down the the transmission to find the reverse switch and do the same with it .
