XJ550 not starting

Time for another "my bike won't start" thread

I've got an '82 XJ550 project bike I picked up a while back and have not been able to get it to start reliably. I occasionally get one cylinder to fire off but its not enough to get it going. On the extremely rare occasions it has started, when I rev the throttle it stays at 3k rpm for a good 15 seconds before coming back down. It also has a tendency to just slow down and stop. Here's everything I've gone through and looked at:

Carburetors - bowls have been gone through and slide diaphragms have been checked. I know it's getting fuel because I smell it on the spark plugs and there is fuel in the YICS channel when I open it.
Spark - Just replaced the plugs. I also checked that the coils are good, and they are reading at the popper ohms. I've also visually checked for spark on each cylinder and its all there.
Valves - I went through all the valve shims and have made sure everything is within tolerance.
Compression - Compression is admittedly a little low (about 95-100psi if I recall correctly, which I know is low, but I'd think would be enough to run. Its been a few months since I checked it, but I think those numbers are about right.)
Cam Chain Adjustment - done
Oil Change - done

It may be worth noting that I've been trying to get it going without an air filter installed. I have one, but haven't installed it yet due to the bike not really running and my sometimes overzealous use of starting fluid.

I really don't know what's wrong with it at this point. Any advice would be helpful.

 

Carb sync.
Definitely at least do a bench sync and then get all the mix screws in the same turns as the one that's firing.

Check that the mix screws have the spring and the o-ring and the little washer also... Don't lose em!

Dry all the plugs, gas torch em, and give it a shot.

 

Also I highly recommend not using starter fluid without an air filter in. Two things, you’ll be running leaner, which could explain the hanging revs and/or die out.

The other is if you backfire, you can get a fire in the air box. I have, and it was not fun, with the thought of torching my child’s bedroom which was directly above the garage. If you do go filterless, have a fire extinguisher on standby. I’m glad I had one right by the garage door.

Now anytime I’m messing around with an aux tank, starter fluid, anything like that, I make sure that extinguisher is within arms reach.

 

Yikes yes fire... Bad.

 

I've since learned from pilot training that that continuing to start/crank the engine should actually suck the fire into the intake and put out the flames, but that's a bit different with a literal firewall between you and the engine/fire. And cleaning up the extinguisher powder was a . . . bear.

 

I used to work at a Ford Model A/ Model T shop, and we always recommended running without air filters because of potential backfires. Solution was the same if they were running one and it caught fire: open the throttle and suck it in

 

It's been over 15 years since that fire, but I seem to recall the can of starter fluid itself specifically warned not to use the product without the filter in place due to risk of fire. Of course I only paid attention to that AFTER the scare. But that's just an old memory, don't have a can handy, and your mileage may vary. What I do know it I probably sprayed WAY too much fluid - filter or no.