I know you said the plugs were wet, but I've heard of people accidentally swapping the fuel supply and return lines. I can't remember if that causes fuel starvation or flooding. Good luck, you'll figure it out eventually!
In fact, your boxer engine does Not need any "Backpressure", it is a pretty usual confusion, what they need is to join the Left head's exhaust to the Right head's exhaust, in a "Y" shaped pipe (other designs works as well) in order to gain the proper Scavenging effect. So, your exhaust needs Scavenging, not Backpressure.
Kind Regards.