Working on riding mower for neighbor, 21 HP Intek single cylinder. Several years ago I put a new magnatron on it and it worked great. It failed a couple months ago so I replaced it with one I ordered from Amazon, supposed to be for that engine. It lasted a little over a month so got another from Amazon, different brand and it only ran for about 15 minutes. I have one ordered from another source that is supposed to be equivelent to b&s part. My question is there anything that makes these fail or did I just get bad parts? I put new spark plug in with coil change a couple months ago. Magnet on flywheel seems to be good.What should the air gap be set at? pulling my hair out!!