Intruders-Alert

My 1999 1400 has sat for years. It was wrecked and I just got done building a new front end for it, updated a few things to my taste, and finally got it running. But it would only idle. I pulled the covers, tank, etc and found the air filters were falling apart. They just crumbled when I touched them. Pulled them out and wanted to see if it would come up off idle. Tried starting it and rear carb filled the air box with fuel, backfired, and made a temporary fireplace under my butt for a minute. Figured it would run better and come up off idle without the restriction of those nasty old filters. Guess not. I’ve read all kind of things about air filter issues with these bikes- so my question is would you have expected this? Does it require the resistance of the filter to maintain correct vacuum? Guess I’m just hoping for reasons not to pull the carbs

Without filters it won't run well. You need vacuum to lift the slides.
Since you say they crumble on touch, my guess is that you sucked crumbs into the carbs.
Spewing gas into the airbox indicates a stuck float valve, possibly dirty fuel.
Since it has been sitting, I suggest draining the tank, and draining both carb bowls.
New fuel with some seafoam, Install new filters and try again.
You may get lucky, but I am afraid a good carb cleaning is in order, followed by a sync of course.

The fuel was nasty. I drained the tank and put a fresh gallon in, but I know darn well what that float bowl probably looks like... Guess I’ll order up some gaskets and pull the darn things out. Was just really hoping to take my wife for a ride before her surgery, but I’ve now fallen behind schedule. It idled so good, I thought I’d gotten lucky! Oh well... it is what it is.