I have a small generator with a B&S engine. It only runs with choke. All the carb passages are clear and gas supply is ok. There's no air leaks that I can find. Compression is only 55#. I filed the valve stems a bit to give some valve lash, .009, and lapped the valves. I put on a new head gasket, but compression remains low. The cylinder is not scored. Do you think there's low compression due to ring wear? Why would this cause it to run only with choke on?