  We theoretically address the problem of cubic B20 helimagnets with small
concentration ${c \ll 1}$ of defect bonds in external magnetic field $\bf H$,
which is relevant to mixed B20 compounds at small dopant concentrations. We
assume that Dzyaloshinskii-Moriya interaction and the exchange coupling
constant are changed on imperfect bonds which leads to distortion of the
conical spiral ordering. In one-impurity problem, we find that the distortion
of the spiral pitch is long-ranged and it is governed by the Poisson equation
for an electric dipole. The variation of the cone angle is described by the
screened Poisson equation for two electric charges with the screening length
being of the order of the spiral period. We calculate corrections to the spiral
vector and to the cone angle at finite $c$. The correction to the spiral vector
is shown to be independent of $H$. We demonstrate that diffuse neutron
scattering caused by disorder appears in the elastic cross section as power-law
decaying tails centered at magnetic Bragg peaks.
