backfire through the carb.

This has been the achilles heel for most of the after market more affordable rockers, while the rockers are probably plenty strong for what you're doing, the adjusters are garbage.
I bought a set of off brand needle bearing rockers and the 1st thing the sales person said to do was change the adjusters and make sure to use the right shafts, having done all that I have a reliable trouble free set of rockers.