how do you check the air gap with an under the flywheel coil?
Good question... I didn't realize it was behind the flywheel on the G series... it's tough to figure out a way without the motor in front of me but im thinking! Lol.
When a coil is snug but not tight the magnets will pull it into them therefore if you could tape a leaf of feeler gauge (or business card as Mckutzy mentioned) on the inside of the flywheel it should auto set. As long as you carefully turn and remove the flywheel after getting it (the flywheel) loose you should be able to tighten the coil back down and hopefully be good!
There's probably a better procedure but I like"winging it"! Lol.
*edit* I just had a look at a picture of one, they don't look like they have much if any adjustment available. If you have the flywheel off again maybe you can confirm that? Was there any rust on the magnets? That will hinder spark.