Grit or a worn needle valve; the float, which controls the gas level in the float bowl, allows continuous flow of gas into the float bowl, and out the hose, because of a piece of grit between the rubber/viton tip of the needle valve, and the screw-in brass seat it sits-in; or wear of the needle valve. You also need to check the brass seat for the needle valve, to see if it's worn. You buy the seat and needle as a pair (part #36). I use either OEM carburetor parts, or I've also used K&L.
https://www.ebay.com/p/182601278?iid=313056771578 K&L carb kit $23.75 including shipping, for one carburetor
Might as-well check the float level while you're into the carbs. And, since you're in-there, first take a look in the gas tank, use a bright light, or a borescope (they have inexpensive accessories for your cellphone to make it one) to look everywhere you can in the gas tank. You're looking for bright, shiny metal everywhere. If you see dark sediments or rust, you're due for a tank removal and thorough cleaning and possibly the use of a tank liner product like POR-15 or similar. If you find that necessary, follow the directions
exactly, or your results will be less-than satisfactory.
If you do find rust in the tank, don't say, "hell, I'm just gonna put it back together and run it, mebbe change-out the gas filter," because
that's not going to fix it, and you're just in-for a boatload of frustration, chasing your tail, and poor operational performance. You've already found that to be the case, right?
"Fuel appears rusty."
I would also pull the jet block, remove the pilot jet (part #42, to remove it calls-for a thin, long, narrow screwdriver blade) and the other jet in the jet block (part #44, called the main bleed pipe), and ensure both are clean and clear. A good carb soak and passageways blow-out to follow. Have on-hand four new jet block gaskets (part #35) because they usually tear removing the jet block. The carburetor bowl gasket, a thin, shaped O-ring usually does-not need replacement, unless it breaks. Removing the jet blocks requires you to split the carbs into front and rear pairs; you do-
not need-to separate the carburetors into four separate pieces. Take lots of pics, to help you recall the relationship of parts to each-other, before and during your disassembly. Use a piece of cardboard, and punch holes into it for each fastener, use a fine-point Sharpie pen to label everything. A roll of narrow masking tape, to fasten parts like shaped pieces or linkages from the disassembly to the cardboard will help you to keep-track of everything, and ease your reassembly. The throttle springs and linkages
do-not need to be disassembled other than to remove the tiny circlips on the rod holding the choke linkages together. Be very-careful when trying to remove these, as they like to suddenly fly-off the rod/linkage, and play 'hide & seek' with you. A pair of locking hemostats or forceps, or a good pair of needle-nose pliers with small tips will help you in removing these tiny circlips, and not-losing them.
Soaking the carbs will probably fill the four chambers which help in starting the bike, by providing enrichment of the fuel/air mix. Those four chambers have a brass piston and nut, with a rubber boot and gasket, referred-to as a 'starter set', part #20 in the fiche. Use a small open-end wrench to remove the brass nut which screws-into a small cast aluminum housing piece that holds the enrichment piston 'starter set' in-place. Take a look at the fiche for part #20, and the cast-aluminum housing that it goes-into, tell-me what is that part number for that small cast aluminum housing piece? Trick question: there is no part number for it on the fiche, as it is no-longer available! Be careful working with them! Use something like WD-40 to flush-out and to lubricate the 'starter set' piston and housing, insert the 'starter set' piston into the small cast aluminum housing piece, and snug-down the brass nut. This prevents you from having to disassemble the rockers and the shaft they are attached-to. The top of each of the 'starter set' brass pistons clip-onto the forked rockers, to activate the enrichment circuit for starting the bike. On the left side of the carbs, upon reassembly into a block of four, there is a bent-rod which connects the shafts for the starting enrichment circuit. Make sure that it's installed to look-like a frown. Consult the fiche if you have any-doubt about its positioning. Check that all-four 'starter set' brass pistons move together, and that they open fully, and close fully. The bent-rod can be bent to adjust the pistons' movement, as-needed to ensure full range-of-motion, and that they all close-completely. At the enrichment lever you can use enough torque on the screw to allow the "choke lever" to stay at whatever point you wish, don't over-tighten it.
If your bike is 20 years-old or more, I would consider replacing the molded-T rubber hose (part #65) which routes gas to the quartet of carburetors. These become brittle and hardened with age, especially if the bike has been stored outdoors or is in an area where there is a lot of ozone (smog) in the air, which causes rubber deterioration. It's not-uncommon when trying to manipulate a 20+ year-old rubber piece, that it cracks or tears. Having one on-hand before you tear-into the carbs is a good idea.
This sounds like a lot of b.s. to have to do, because of a ^$#!!* piddly-ass gas overflow hose peeing gas onto the ground, but if you have a gas water heater in the same place you store your motorcycle, you could end-up with a house fire. Gasoline vapors can travel 30 ft to a source of ignition like a natural gas water heater, or a gas furnace, and ignite. I'm a firefighter, I've seen it (now retired).
To get the bike running when it's back-together, you need some-sort of a carb synchronization tool. The analog gauge systems are cheapest, the electronic ones are most-expensive, the liquid ones cost-more than than the analog gauges. If you don't have one, even an inexpensive analog gauge set will get you the settings done which you need, and it will pay for itself the first time you use it.
What I'm telling you is that you can approach it from the viewpoint of, "I'm just gonna use a long socket extension, and tap on the leaking float bowl, and hope-that the piece of grit causing me all this grief is gonna dislodge itself, and come out the overflow tube. Then the ^%%$!* float needle valve will seal again!" It may, it may-not. Answer me this: where did that piece of grit come-from? A rusty, gunky tank? Then it's time to perform this maintenance.
One of the most-common things affecting our bikes is the pilot jets in the jet blocks becoming plugged with sediment or fine particulate material. You see in the one pic, where a pilot jet has a single strand of stainless steel wire sticking out of it. That single strand is a bit over 0.01" in diameter. It doesn't take much to plug those four pilot jets, and the ones on the left side seem to clog quicker, because most people use their side-stands, and the crud migrates to the lowest point, the left-side carburetors.
Yeah, I know,
this is crazy, all's I wanna-do is stop the tiny hose from pissing-gasoline! Unfortunately, a leak in the wrong room with a gas-appliance could burn-down your bike storage area which just-may be your home/garage. Search-out what needs to be done, order the parts, and label those parts as you remove things.
https://www.ronayers.com/oemparts/a/yam/50045c0ef8700209bc7942f3/carburetor
One carb synch tool, I've used one similar to this for nearly 40 years:
https://www.amazon.com/WIN-MAX-Carburetor-Synchronizer-Adjustment-Tool/dp/B075YSWZ21/ref=sr_1_29?crid=3B1Q8EMWL35WB&dchild=1&keywords=motorcycle+carburetor+synchronization+tool&qid=1588561000&sprefix=motorcycle+carburetor+synchronization+,aps,177&sr=8-29 Amazon $47, free shipping
I also have a set of Motion Pro Carb Stix, there really isn't much-difference using either the Motion Pro Carb Stix or the analog gauges.
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