YAMAHA QUAD NEUTRAL/REVERSE LIGHT!

hey guys, i have a yamaha quad, its not a grizzly (although i have one of those as well) but i have a quick question. its a 1987 Moto-4 350…i had the motor removed to replace the cylinder head. it was out of the frame for 2 years and i finally got around to putting in back together…but now when i start the bike the neutral light comes on (in neutral like it should) but if i idle it up, or push the throttle, the reverse light will come on. its not really a big deal, the bike works fine. just wondering what i did wrong when i put it back together?…theres a battery on the bike but its dead…would a dead or weak battery cause this? any help is appreciated thanks!