30% of the way there...

I’d check to make sure all the enrichment jets in your bowl are clear. If not I bet your pilot jets are the problem

 

The carburetors have been rebuilt, they're like brand new.

 

Is there a carburetor guru out there that knows the solution? It'll only fire up after I've poured fuel directly into the airbox. Then It'll only run full throttle. The RPM variance you hear is from me flipping the kill switch on and off.

 

If they were like new you wouldn't be having this problem.....
Take them out, strip and inspect. This is where you'll find your answer.

 

I've removed the rebuilt carbs, and everything seems perfect. So I pulled the intake boots...

 

She'll start if I spoon feed her, but then will only run as long as I keep the revs above 5K. I pulled the fuel line and cranked it over, and she was spitting fuel, and the bowls have fuel in them... Hmmm

 

Let's see if it'll work...

 

It's a really pretty bike! Bummer that it's fighting you now, but you'll get it. Hold off on that chipper! Are you certain the enrichment jets in the bowls are clean? Did you get the Old Faithful geyser with carb spray to come out the top when sprayed in the bottom?

 

Done.

 

Lol. Take a deep breath and a step back my friend, you've done some nice work, don't spoil it now.
Work through the issue carefully and methodically.
Your engine sucks air and any fuel it can past the butterflies - are they all aligned, and very nearly all closed?
Can you open your bowl drains and get fuel out? measure how much from each one.
Where are the mixture screws set? 3 turns out should be a good start.
Did you check the fuel enrichment jets in the bowls?
I could go on, but it's for you to think about this, I've just given you a start.
Edit to say, I've skimmed through your 8 pages. One thing jumps out at me - you bought a carb rebuild kit from an ebay seller? If so, dig out all the jets you removed, one by one check they are the same as the ones fitted - by looking through them, but ideally you should clean and refit them, all of them. It's been said here time and time again, original Yamaha or JX forever parts are the only ones you should put in your carbs....

 

When you have it running like that are all 4 pipes hot?

 

I ended up not using the fleabay kit because it was shit.

Absolutely EVERYTHING is EXACTLY as it should be. And the pile of shit refuses to run.

How is it not pulling a vacuum without any vacuum leaks anywhere?

 

I see no reason to continue to pour time and money into it.

 

Patently you are wrong. And the funny thing is, you have made the error, but you blame an inanimate lump of aluminium and steel for your failure.
Does that hurt, well it should. Time you grew up young man.
Start from scratch, cam timing, carbs, spark (although I suspect provided the timing is correct your spark is where it needs to be).
If you bothered to ask, there are people on here who would sell you a rack of carbs, all set up and ready to run, just not running synched. Just ask...

 

Found the problem.

Only 80psi across all four cylinders. (Should be 156psi) So it's not creating enough vacuum.

It's the shitty internet head gasket.

No one has the good ones, dealers can't even get them because no one is making them anymore. The only head gaskets available for it anymore are Chinese shit.

 

Hard to imagine a head gasket causing low numbers on all cylinders, but maybe as stated above

 

Was the throttle full open when you did the compression test?

 

Surely if the gasket was that bad they would not all be 80psi? Len @chacal I bet he has the right head gasket. Full open throttle check. You ain't far away now.

 

Hmph. Suck squeeze bang blow...
Don't see how a lack of squeeze would affect the suck at low rpm?
Aand, if you're having to give it fistfulls of throttle to make it run you won't have any vacuum anyway.
Cam timing...