Motor starts and runs till i put in 1st gear then it will die.if u start out in 2cnd it will not.will run till u drop in 1st

It could be due to:

Fuel restriction
Low engine compression / worn engine components
Worn spark plug
Plugged air filter

never do that;
now you get to dry all that out now.
do not use pressure washers.
on any engine, and for sure electronic under hood
and now all spark parts and fuse boxes soaked.
dry it all out, use hair dryer . (safe it is not too hot)

Perform ODBII scan. Look for speed sensor fault. Trans has to read speed to know what gear to engage. Could also be a governor problem. Try shifting from low to high gear manually

Riding Mower Dies When Put in Gear ' ThriftyFun

www.thriftyfun.com/Riding-Mower-Dies-When-Put-in-Gear.html

It starts fine, but when I put it in gear to go forward or reverse it dies. It also dies ... The engine on my riding mower will start and run while the clutch is pushed in.

Starts and Runs but Dies When put in gear - YSR 50 ' South Bay Riders

www.southbayriders.com > ... > Community > Maintenance and Repair

Aug 25, 2010 - 15 posts - ‎4 authors
The bike will start and run perfectly fine. However, the moment I put it into gear it dies. ----------------- How we got to this point:...

Engine dies in gear, runs in neutral - Hot Rod Forum : Hotrodders ...

www.hotrodders.com > Tech Help > Engine

Jan 4, 2010 - 12 posts - ‎10 authors
But if I drop it into any gear besides neutral or park, the engine will ... In this case, it shifts fine, and on time, however acceleration is ... start. the engine idling rough and wanting to stall when put in gear is the indicator to me.

Could be fuel filter,air filter,tune up,wires,coils.low idle too low.

what year and motor

faulty trans shift solenoid for 1st gear

Check the electrical switch in the clutch lever, most bikes have one as a safety, so you cant start it in gear, unless the clutch is in, could be a faulty switch, also do you have a switch on your side stand, sometimes these can be faulty as well.

I suggest you take it back to whoever did your transmission.

Did you change the fuel presure regulator located in the throttle body?