Cut It Close,
The original spindles where completely flat with no exposed shaft and the blades bolted directly to the bottom with no spacers. If you have these spindles and the blades are spaced then the blades are too low in the deck and in their relation to the baffle allowing the blowout. The spindles that have spacers will have the shaft exposed between 1/4" - 1/2" on the bottom of the spindle depending on which version it is. Valves should be checked/adjusted every 300 hours and could be the cause of the back fire at start up.