Bike Bogs Out When In Gear

[sundayrider1]

Just got a kdx that was in storage for a very long time
After some cleaning, I was able to get it running and moving up to third gear
Parked it and started to do more fixing.
Did not, however, look into motor
Bike starts and idles extremely well.
Upon shifting into first gear and attempting to go forward, motor dies out
What problem might this be

 

[Okiewan]

Does it jump/lunge when you put it in gear?

 

[sundayrider1]

No
Idiling smoothly, rev it up a little
back to idiling
pull in clutch, downshift to first
release clutch slowly, bike bogs and dies