Will only turn over for a moment...

I need some wisdom (utter novice here)... 1980 KZ440 sat for last 15 years... I can get it turn over a few times, usually only when I inject a starter fluid through the air intake. I did a minor carb clean but could some smaller jets be clogged? I have spark and around 100 lbs compression (which I know is low). I advanced the timing as far as it would go which I think is now correct (point gap is good and ohm meter flickers when passing the F mark).

I have some clear tubing standing in as a fuel line and I can see plenty of air in the tube so no vacuum... Problem?

I think the carb bowls are filling because the overflow drips every time I squirt in more starter fluid. I think the problem is fuel delivery but I'm stuck. No difference opening or closing the choke but more likely to do something when the choke is closed.

I removed the old air box so the carb does want to hang back a little vs the slight downward slope. Carb boots probably old and leaking. But I don't know...
Suggestions?

A compression test should be done on a hot engine but I realize you don't have that option. More importantly you must have the throttle wide open or you will get a low reading.

After 15 years of sitting you almost certainly need a carb rebuild and your symptoms support that notion. Fuel bowl levels should also be checked. Both of these things are critical to getting an engine to light up. If fuel was left in the carbs/bowls then I'm 100% sure you need to rebuild them.

It is very hard for me to see fuel flowing even in clear lines. Might just be my old eyes, but it is very subtle.

Most of these engines run very poorly with the airbox removed. Fix what you know is wrong and if there are other issues they will be more clear with the other items fixed.

You need a service manual to guide you through this and you will be fine. Most here much prefer the factory service manual. I do too.

I should have added, what does the inside of the tank look like? Have you shined a bright flashlight in there and had a good look around. If there is significant rust, don't bother cleaning the carbs until you clean the tank first. Otherwise a few cranks and it will slam your carbs within seconds. Fresh fuel is a must.

Thank you! I had the tank stripped and sealed so I think I'm good there. I have the purple shop manual available online so I'll start looking into a carb rebuild.