There is a weird method to check the spark plug firing on a waste spark like yours. I think you have to put the two plugs together with a hose clamp, threads to threads, and ground the pair, or something like that. I think the method was on the sds.com web site. If the plugs are in fact not firing, I think your tach would be reading 1/3 low if it's the ign module, because the tach would not be seeing the coil ground path break.

Not responsible for advice not taken...