I bought a Snapper Rear Engine Rider model 301223bve with a 13hp briggs and stratton from a gentlemen that was downsizing to a smaller yard. I've used it for a couple months and it always fired up on the first or second pull of the cord (it is equipped with electric start, but previous owner said battery was bad). I changed the blade over the weekend because I noticed my grass had rough edges. I had to tilt it on end to do this and it probably took about half hour total because I got distracted in the process. No gas leaked out because I closed the vent on the gas cap. A little oil leaked out of the oil cap. I was told it is no problem standing these mowers on end, but I'm starting to wonder if I got some bad advice. I tried to start the mower the next day and nothing. After about 20 pulls I removed the spark plug and noticed it had some oil on it. I pulled the cord a handful of times to try to get the oil out of the cylinder but nothing came out. I put the plug back in and proceeded to drink beer and leave it for the night. The next night I tried to start it again and same result. Pulled the spark plug again and did not seem to be any oil this time. I changed out the spark plug to a new one and tried to start and still did not start. I'm at a loss and am not sure what to try next. I replaced the fuel line and filter the day I bought it so I don't think the fuel filter is the issue. The air filter is in good shape and the gas is less than a month old. Any help on this would be greatly appreciated.