Robert M. Kimbrell wrote:
Are the radials a high impedance on 20 meters with a 33 foot
radiator?

No, 25 foot radials laying on the ground are not a
high impedance on 20m. One side of the feedline sees
about 1200 ohms, the other side sees an impedance
probably much lower than 1200 ohms.

I can disconnect the radials and have an endfed zep and it
works ok.

With an unbalance of the currents. One side sees about
1200 ohms, the other side sees an open-circuit. An endfed
Zepp requires 1/4WL of feedline. Is your feedline an
odd multiple of 1/4WLs on 20m? Probably not unless you
designed it that way.

Connecting the radials makes the antenna perform better and the
tuning is different on the tuner in the shack.

Either way, you have common-mode currents on the feedline.
The radial configuration doesn't require 1/4WL of feedline
and may just be more efficient.
--
73, Cecil http://www.w5dxp.com