If the oil was way overfilled when you got it, chances are that it has filled the exhaust up and the cylinder, if this is the case just use it once or twice to clear it out.
Try having a tinker with the screw on the front of the carby while its running to change the tuning, I have a rover identical to that out on my back lawn at the moment that had that problem among others.
Other problems with my identical rover included, the exhaust valve seat had come out and I had to push it back in and then use a punch around it to clamp the metal against it. Then the exhaust valve wouldnt seal as the seat had worn down over the years, After some quick thinking I pulled the valve out and took a good half mm off the valve stem with the bench grinder and put it back in.
After I had fixed everything I started it up and it runs nice, just TOO BLOODY LOUD, then I found the exhaust laying on the ground next to it where I pulled it off.
