sparkplug not firing 82 kz550

Check the spark on #2 by grounding it outside of the engine and have a great spark. But when installed #2 does not fire exhaust is cold and gas is come out of exhaust by the slip-on.

This started after I put the stock jets back in. What could be causing this?

Try swapping 2 and 3 plug leads first, then the plugs, assuming the condition remains then you have a fuel supply problem in the related carb. Either too much or not enough. Check your float level in that carb using a clear pvc pipe U tube manometer.