The motorcyclist, a male in his 30s, was badly hurt after the collision between a black Ford Fiesta and the motorcycle at around 3.30pm (June 30).
Any witnesses should call Lincolnshire Police on 101, quoting incident number 313 of June 30.
Earlier the same day, another motorcyclist was also seriously injured after a crash on the A1084 at Owmby, near Caistor.
The rider was taken to hospital following the collision with a van which was reported at 1.30pm.
Any witnesses to this collision should call Lincolnshire Police on 101 and quote incident number 250 of June 30.