Having bought & used a few timing lights thru the yrs. I can tell you they are not all created equal. But, the first thing I'd do is pull a plug wire while running & see if that sucker won't jump a big fat blue spark. Now, if you see you have a good spark, all you need to do is look for a good light. I have a Proto I bought new around '70, non inductive, & a Cal-Van I got on ebay a couple yrs. ago, it's even older, both have a strong light. I've used some quite expensive shop ones that weren't as bright. But, I have no 6V. motors so can't tell you for sure there.
Have you checked the primary voltage to your coil while running? You could jumper off the batt. to it & give it a full 6V. while timing.
Doug
