I have a VERY STRONG preference for a spark tester that forces the spark to jump a calibrated gap instead of making a light-bulb glow.if you do not have one of these or know some one that has one of these I would get one easy safer way to check for spark at spark plugs https://www.ebay.com/itm/374223151089?epid=1128500261&itmmeta=01HYPG6CB6ARZ0FDMNWWK35CR5&hash=item5721701ff1:gLwAAOSwLhdjAswt&itmprp=enc:AQAJAAAA4MhnnRKo/Q9JQssPrU2caYf1mFbGUf/NDYqOIElXgAVP/WgEJe9uvf19oVuMrHkMekn/bv7LVZMgCc3UwP3Qfjq5VP9CHG6Z5/4gdXz+qYnIQ1aw4e7KXGNGtA9RhOP3ZftqniDrIIpDoPgOKo89DFs3f4YxLrwzDAMekEYFj3WgBt636j1qKrLlgh+uFf7eJpbThumupfTQTspQs/MFMv0QCUmCyD6MtzvBn6o--roaHhJ/0EF1ZQBA3DFOqVhZa24QvEJQzhvKA/ZFvzY/njyQhmD8adzETaTWvZNCQh4m|tkp:Bk9SR9rFmdD1Yw
The whole purpose of the spark is to jump the gap at the end of the spark plug, when the plug is under compression pressure. A weak spark can make a light-bulb glow, but perhaps not jump the spark plug gap when the plug is under pressure.
A spark plug in open atmosphere doesn't load the coil enough. Therefore, a spark tester calibrated for HEI, where you can SEE the spark jump, is the better way to test coil power.
There's multiple designs of spark testers that have an actual spark gap, this is my favorite style:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it