If it runs ok otherwise, you probably just need to turn the idle up a bit by turning the slotted screw clockwise. Reverse causes increased backpressure on the exhaust.
If you put the shifter in reverse and try to turn the flywheel by hand, will the prop turn in reverse? If the flywheel appears locked in reverse, may be an issue in the lower unit.
i have 1977 johnson 140hp that dies when i put in reverse, other wise runs ok, what to do?