Check here for the welt foot for your machine.
This company is in Calif. but they may ship overseas. Don't mess with the timing on your machine, leave that to a professional. I suspect thread may be your problem. Find a source for B69 bonded thread polyester or nylon. Check your needle size for the thread your using and the general rule of thumb is the bare needle should slide down the thread by it's own weight. Observe the thread as it feeds into and out of the tension washers, is it twisting the thread, or unwinding the thread? Bonded thread will feed smoothly with consistant tension. Stay away from vylon which is not a twisted thread......tension will not be consistant. Is the needle installed correctly? Go back to topstitching some scrape pieces and adjust your tensions there and one final question.........which side of the needle is the bobbin, left or right, or below? If your still having problems let us know.