300/440/550 650sx starting problems

I have a 87 Kaw 650 and wont start. It gets gas and has spark. When it did run, it ran I shut it off then started it back up, it ran fine then shut it off again. When I waited a lil I tried to start it back up and nothing, just cranked over. The spark is there it is just a weak one and sometimes it cuts out on the back cylinder. Any ideas?
 
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