As John said, there are any number of engines from that period so it'd be best to know just which one it is. If you're not completely sure then go to [link=http://www.osengines.com/history/ostimeline03.html]this[/link] link and find the one that matches yours. In general though, and without more information, there's a couple of things common to all these engines that could cause your symptoms. If the liner is in back to front or the piston fitted backwards (or both ) you'll have all sorts of problems. They'll run but barely. For the liner, the exhaust port is the one that opens first as the piston is coming down but you'd really need to take off the head to see because the baffle on top of the piston blocks your view towards the transfer port. For the piston, the baffle has to be on the side furthest away from the exhaust port.

None of the engines from that period should be run using common RC fuels.