1986 Honda Fourtrax 350 4×4 issue

I am new to the forums and I am sorry that if this has been asked and answered.

I have a 1986 Honda Fourtrax 350 4×4. It starts right up and idles. But when you give it throttle it sputters and black smokes. Like it is hitting the Rev limiter It just started. I was mowing with it and all of a sudden it wouldn’t pull my hill. I have own this thing since 86 and oly have put a handful of coils on it. Any advice or suggestions would be great

Thanks guys