Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Welcome to JUST ANSWER small engines, let me try to help you !! I don't know it all, but maybe together we can get you a satisfactory answer!
Cylinder with the black look has weak spark or the valves need to be checked. Or the carburetor, has a leaking jet in it. This carburetor has to jets, one for each cylinder. See the link below.
I see question is still open, did you find problem?