As I said in a previous post, any bits in the carb bowl larger than .4mm (.018 inch) will not fit through the idle or slow jet orifice and block it. This will starve the cylinder affected for fuel at the low throttle settings. The main jet is 1mm (about .040 inch). So, particles bigger than that will block the main jet. And then, no gas will get to the cylinder at any throttle setting.
Sometimes the inside the fuel lines will deteriorate and pieces will break off. Examine the fuel lines and see if you can llightly scrape the inside of the hose. If pieces come out that look like what you found in the carb bowl drainage, then you've learned the source, and you need to replace the fuel tubing.
Unless your are eager to dismantle the carbs and examine the jets. There is a couple things that may relieve you of that task. Just like peas will sometime stick in the end of a drinking straw and sometimes they fall off when you stop sucking, so too, can jets sometime clear themselves when the carbs stop sucking fuel. Since you have found significant particles in the carb bowl, it is best to get as much of that stuff out as you can. The auto parts store sells aerosol carb cleaner and they come with a tube you can insert in the spray nozzle. Fit this nozzle into the carb bowl drain hose and give it a few shots to flush out what you can. Once it flushes clean, there is nothing to lose but to put the bike back together, feed the carbs clean fuel and flush the carbs with gas from the tank out the carb drain holes until you get nothing but clean fuel out those drains. If the jets were/are loosely plugged, this might do the trick and your carb woes are cured. But, in any case you've nothing to lose now by try to run it. Charge the battery up good, turn off the headlight and start it up. Check the exhaust head pipes for even heat by flash touching each pipe. You won't get burned if you do it quick enough. If some pipes are cooler than others, then there are still some jets plugged up. Sometimes engine vibration eventually allows them to clear. Sometimes, soft debris can be sucked through the jets, and clear them, by creating a very strong vacuum in the carb bore. This is done by reving the bike to high RPM, rapidly closing the throttle and choke at the same time. Then open the choke again after the RPMs have fallen. With practice, you can get several cycles of doing this without stalling the engine completely. If this works, there will be a change in engine sound. And, then the exhaust headers will all be evenly hot. Set your idle and go ride. If it doesn't work, you'll have to remove the carb bowls for the cylinders that have cold pipes. And, clean the jets more vigorously. If the o-rings inside are old and brittle, you'll want a fresh set on hand for reassembly to avoid leaks and other odd run behavior. If you need it, I can try to describe how to fit the main jet, spring clip, and fuel bowl on at the same time, with the carbs still on the bike.
Cheers,
