14.5 ohv briggs

Need help with a Murray riding mower I just got, it's been sitting for 5 years,cleaned it up by cleaning plug and wd-40 in cylinder, when I started it, it cranked but was sluggish and seemed to get stuck I then took valve cover off and plug back out and with the plug out the motor cycled good and both rocker arms were cycling but when I put plug back in it had a hard time cycling I was spraying the carb with b-12 but nothing any ideas guys

A couple of things come to mind. Possible bad battery. (Is it the right one? To small a battery won't provide enough power to turn over well) Possible bad battery cables or connections. Possible bad starter. So, check/clean all battery cables. If no help, you might try hooking jumper cables up to your car maybe and see if it turns over better then. If it is better, you might have a bad battery (even new batteries have problems sometimes) or possibly too small. If all else fails, you might need a new starter. A starter can turn the engine over OK under no load conditions (with the plug out) but draw too much power under full load (plug in) to turn engine over well.