Stalling when down shifting

I am restoring a 2006 LTR450 that wouldn’t start when I bought. I replaced the pump, injector, regulator and did the swirl filter delete. It starts now but takes some cranking but after started it will idle all day. I rode a lap around my yard and got it into 2nd gear and when I down shifted to 1st gear it immediately cut the engine dead.
Has anyone has a similar experience? If so what caused it?