Key to effective troubleshooting (fast and inexpensive) is non-invasive and non-interference
testing to come up with numbers. Not changing anything in system and not introducing additional errors. We want to leave everything alone and test small sections at time until we positively identify single problem.
1. BIG-PICTURE: great, you've found problem is lack of petrol from carbs by squirting directly into airbox. Minimal invasion and changes to system, you put air-filtre and cover back onto airbox and everything's back to way it was.
Now we just need to test individual parts of petrol-flow starting from source to destination with
minimal interferences or changes.
2. TANK - how do we know for sure petrol is leaving tank? One test was riding with cap open to verify there's no vacuum build-up, great, that passed. What about actual exit at bottom? If we remove petcock completely and dump petrol in filler, we should see it drain out bottom hole right? Let's assume that will function properly as exit is plenty large and won't clog up overnight.
3. PETCOCK - next stage in petrol flow. We want to test petcock in-place and operating it exactly same way as engine would under proper operating conditions. That means applying vacuum to little hose and seeing if fuel flows from bigger outlet hose. Unplug outlet fuel-hose from carbs and aim into measuring cup.
3a. Crank engine for 10-sec and see much petrol flowed. Dump measuring-cup back into tank and point outlet hose into cup for next test
3b. Apply vacuum to little vacuum-hose either with pump or by sucking on small hose. Be careful as you might get mouthful of petrol if petcock diaphragm is broken. Which in itself is successful testing of petcock, but not very tasty. If you don't get mouthful, then diaphragm may be OK. Continue sucking and let petrol flow for 10-seconds and verify how much you've collected compared to 3a
I had similar issue with bike dying after 30-seconds on track. It would idle and warm up just fine in pits. Then after I got on track, it would die exactly 1/4th way into lap. I pushed it back to pits and started it up. It start and ran just fine for several minutes. Went back out on track and again it'd be dead 30-sec 1/4 lap in!! :
Logically, it appears to be petrol-flow problem... must be carb issue... maybe...
I did petcock flow-test and found it! AHAH! I had used extra long vacuum-hose between carb and petcock so it would be easier to re-install tank. But... this created an extra loop of vacuum hose that I draped over backbone of frame. Then when I lowered tank, it pinched this loop of hose and restricted vacuum going to petcock. This lower vacuum opened petcock just enough to flow for idle and sitting around pits. But not enough to handle WOT.
So... you need to confirm that your petcock is flowing sufficient petrol in ltr/min before doing ANYTHING else more invasive and possibly damaging to your bike. It's simple, look at picture above, disconnect fuel hose from carb-inlet and aim into measuring cup. Apply vacuum to little port on petcock with pump or by sucking on it. Time flow for 10-sec and stop. How much petrol did you collect?
4. FUEL HOSE - between petcock and carb. We can remove hose from petcock straighten it out and look inside. Should be clear from end to end with no obstructions. Can also floss it from end-to-end with test-tube brush to ensure it's truly clear.
5. CARBS - symptoms of dying overnight doesn't fit issue of dirty and clogged carbs. Which occurs slowly over time, months & years. So most likely issue with petcock or fuel-hose, see above
Note that none of these tests require completely disassembling bike down to last nut & bolt. And doesn't require trouble-inducing re-assembly. Do tests to
positively identify problem area before going to desperate measures!