Hi are you trying this with an auxiliary petrol tank or pipes to the bikes tank , and if it's the bikes tank does the petcock have a vacuum pipe on it

 

It's a '95, so no throttle position sensor. I'm using an aux tank suspended slighting above seat height feeding directly to the fuel pump. It has clear line, and I can see (at least initially when there is some air in the line), the fuel pump fill the line and push fuel to the filter. When I remove the hoses that T off the feed from the fuel filter, they are full, as are the float bowls (as evident when they are drained through the drain screw).

I went back over the pics I took when the carbs were disassembled, and noticed 3 ports. I'm wondering if these regulate or facilitate fuel flow (and if their blockage could affect things) since nothing appears wrong with the fuel side of things.

 

Air jet feeds from the Airbox. Is the Needle Jet at the correct height, and what sizes are your jets, the Pilot Screw should be 2 3/4 turns out from bottom, main jet 115, pilot jet 40

 

Yes, 115 main, 40 pilot. As far as needle jet, it's stock with a single washer at the end (in the holder) and all look to be perfect shape (and consistent with each other height wise). Not sure what exactly the correct height is (but it ran flawlessly prior to it being stored for 1.5 decades). Pilot screw was returned to 2 3/4 turns after carb cleanings. Have also tried 2.5, 3, and 1.5 turns, with little effect.

My latest theory is the float chamber is perhaps vacuum locked (if that's even possible) restricting or preventing fuel from exiting via the chamber through the jets, so I'm ensuring (again) that the vents pictured above (which lead to the jet holder tubes & float bowl) are free flowing.

 

It does seem that you are being starved for air. Assuming you have enough fuel in the bowls, you should be able to rev the bike for a short time even without a tank attached.

 

I believe opening the throttle leans it to the point of quitting. One experiment I did to try and determine if issue was lean or rich:
with the air box removed, I took a piece of masonite (basically a flat board), and laid it across the air intakes. With it idling, I slowly covered more and more of the intake openings. When just about 1/4" of gap remained uncovered, it started to respond to throttle. This lead me to believe that it was receiving no extra fuel (or so little that it didn't match the increased air flow as the butterflies were opened), and essentially leaned it to death.

After I re-verify the air ports located under the intake air horns are clear on all the carbs, I may repeat the experiment to see if anything has changed.

 

Try running the Aux petrol tank straight to the carbs, dont use the inline filter or pump ,gravity will feed, just to take them out of the equation, what condition are the spark plugs in (wet/dry)

 

That is a really good idea... take as many things out of the equation as possible. Also, thank you very much for the PM. I attempted to reply but it said your settings prevent receiving PMs.

 

Question relating to the slides.... how hard should they be to lift up? (I know that's difficult to quantify...) Wondering if the diaphragms have stiffened with age (altho, there are lots of 900RRs out there with similarly aged ones working fine.) Mine take a little effort to lift, but are by no means difficult (and descend back in place in around 1 to 1.5 secs).

I performed a bit of an unorthodox experiment. I took the hose of a small shop-vac and sealed it against the intake manifold side of a carb. As I opened the throttle, the slide lifted. But at full throttle position, it was only 1/2 way up. I've seen a carb testing video of low pressure (low volume) air being blown into a CV carb (air port), and the slide lifts quickly and easily. Would the engine generate significantly more suction than a shop vac? Yes.. getting desperate to get this thing working.

And thanks for all the ideas and suggestions. The support is helping in many ways.

 

When doing it by hand the sliders should have some even pressure pushing against you, but not a difficult amount. When they return it should be a quick even motion with a sound that's a cross between a whoosh and a slurping noise.