Weak spark 84 big red & 200x

[4 wheels?]

My 84 br has been running great but today when my friend was riding it, it just died on him. Had compression, but wouldn't start. I thought it was eletrical because I had just taken the battery out and put it into the warrior because that is being traded on sat.(ill tell more about this and the 200x below.) So I took the battery in the Warrior, put it in the br, the br started and ran great. Br rode home, went to drive it some more, it acted like it ran good for a few seconds then acted like it was low on power and was sputtering a little bit. I checked compression with my thumb, felt good but the spark was yellow and weak. Now it won't spark good(yellow, sometimes only first pull) I think it might be the coil but I am unsure.

Now to the 200x. I am trading my warrior for a 97 mxz 670 and a first gen 200x. Now from what the guy says, the 200x has close to the same problem, but will run good, start easy, other times won't start. He has an extra front fender, triple trees, and a motor with no cylinder. What do you guys think?
Thanks

[Yamada]

First thing to do is change the spark plug. Was is the color of the tip of the spark plug?? Completely black?

[Erics350x]

sounds like a poor ground on the br and the cdi on the x

[4 wheels?]

Well the br runs good when it is cold. When it warms up, it doesn't run right making it seem like it might be the coil right? The tip of the plug is carbon on the outside, brownish tan porcelain and the very tip looks normal colored.