94 evinrude 40hp starts and idles in forward wont idle in reverce

Could be a few things here, one being the fuel. Not sure if your using the 6 gal. Potable tank or built in but get you some fuel system cleaner and run that through a tank of fresh fuel, you should be doing that often to help with the nasty fuel. If you can, buy the no ethanol fuel, any gas motor with carbs should only be running that...

Another thing to check is make sure your motor is coming to temp. Really important for it to run at temp, you get some trash in the Stat and stand it open, it won't allow the motor to reach temp at the slower speeds and it might just be on the edge of to cold and enough the motor go rich enough while in reverse and die. Yes I know at idle in forward it does run, but when in forward the exhaust just rolls out through the hub of the prop real easy. When in reverse the water will add a little more back pressure on the exhaust trying to get out and can be just enough to kill the motor if the motor has a small issue that causes it to unbalance.

Might give these a try, if you run the quick disconnect fuel line, at the end of each day, just unhook the fuel line while the motor is running and on the trailer. Then just let the motor run till it dies... Then your ready to go, running the fuel out from those carbs each time will help, just need to run some good fuel system cleaner in a tank of fuel ever so often.