Dont remove the gas cap;;;!!!! simply loosen it a turn or two. You dont want gasoline splashing around on your yard or you may not have any more grass to mow. Try just letting the mower set still and run for the allotted time.
If that does not fix it, go and get a Honda. I bought my Honda in 1987, and it has never been in the shop, has had 1 air filter and spark plug replacement, still starts on the first pull even after setting up through the winter, and the oil stays perfectly clean... 17 seasons and counting.