can't release belt tension on Snapper 381451HBVE

I pulled the guard , underneath engine on frame (the plate that carries a pair of belt "guides". routed the belt thru the tension idler onto the main drive pully (coming from engine) and even allowing all possible adjustments to the "slack" position there is still so much tension on this belt that cannot start because it's attempting to drive the blades. any ideas greatly appreciated.