I have a 94 klf400b and i replaced the timming chain.i timed it just like the book siad but it back fires thru the carb.any ideas??

Had this machine for 6 years. Has been great for what we us it for. Few months ago got where it had no power. Would not "run out". While riding would only open up to about 1/4 throtle but would never open up. Barely pull you. Even sitting in neutrel and reving it up it would only take about 1/4...