Venom Vader 125cc tach not working
 

All in the title, bought the bike with the big bore kit motor off the frame. Put it on, runs decent, I think it needs a new carb or something bc it feels like it doesn't really have any power and I can't seem to get it to go over 30mph, but I can't tell at what RPM range power falls off. The tach just 100% doesn't work. Nothing shows up on the screen to indicate that it's at 0 rpm or anything, just blank.

Is there a sensor or wire I'm missing or do these just not have functional tachs?

As to the power situation, check both the valve timing and valve clearances. Simple and free. Then look into carb issues. Maybe it's too small, needs rejetting, or the motor needs a better carb.

By valve timing I mean that the marks line up on the sprocket in the head. Just being off one tooth can matter. Never assume someone before you did their job right.