Not that this is going to be any help, but I've used the H/W on my 235....didn't experience any problems with the secondaries. Was wondering if this backfire was experienced under load or just free-winding? Also, make sure you're tapped into the correct vac port on the carb.
I had a little trouble getting the carb to adaptor bracket to seal up....a bit of a miss-alignment with the slotted adaptor holes. Sure sounds like you might have a lean situation. Would imagine a call to Tom L. might be in order.
He'll bend over backwards to make sure you're a satisfied customer.
Dave
