If you think the Boyer is causing kick back, the battery is good, there isn't a volt drop in the supply to the black/red/blue box, and you've tried a box swap. Believe me your ignition is too advanced for your motor. I had two years of hell, oh boy! the pain I went through.
Turned out to be a new strobe light that was guaranteed accurate up to 8000rpm. Well it wasn't, the flash was slow like most other strobes. I've since had it replaced and the new one calibrated to 8k but never had the confidence to use it.
Does your motor have piston slap, pink or rattle? if so retard it in small increments until it stops. Or just try retarding it until it stops kicking back. It cost nothing to try. You might find the tickover will change, that's expected. You should find the motor runs smoother and rev freer if you were too advanced.
You can check if your ignition is OK by plug colour, and heat marks on the plug electrodes. There's a lot of nice sites on plug reading to set ignition timing well worth a trawl.
Sorry for getting on the soapbox the memory and leg still hurt.