Check the float level . what you describe suggests that the fuel level is not high enough to service the idle circuit .make sure that you have adjusted the idle screw as well
SOURCE: 1993 olds cutlass supreme 3.1 multi port loading up at idle
Here are some of the causes of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.
SOURCE: 2007 Yamaha grizzly 660 wll run fine, and then
CHECK VALVE CLEARANCES(INTAKES MOSTY) IS IT ONLY HARD TO START AFTER IT HAS BEEN RUNNING(HOT) OR IS IT HARD TO START COLD TOO?
SOURCE: yamaha 125 grizzly starts and idles when choked but dies when acc
Take the carburetor apart again, remove the main and idle jets and idle mix screw, plus the float and needle valve. The main jet holds an 'emulsion tube' in--look inside where the slide valve moves, you should see a short brass tube projecting out of the bottom. Carefully push down on that tube with a plastic rod or wooden dowel, it should drop out the hole where the main jet was. Chances are that the tube is dirty and needs to be cleaned--look carefully at the series of small holes drilled into the sides of the tube. These small holes deliver air which is mixed into the fuel passed through the main jet. That air comes from a small hole just inside the intake side of the carburetor. The long needle that sticks out of the slide bottom controls low to mid-range speed fuel-mixture. Check that the 'C' clip on the top end of the needle is in the center groove and that the retainer spring is pressed down the 'C' clip. Now soak the whole carburetor with spray cleaner, then blow out all passages with compressed air. Don't soak the rubber-tipped needle valve from under the float. On reassembly, set the idle mix screw 1-1/2 to 2 turns CCW from CW stop. Adjust for best idle after you get it running and warmed up. I hope this long winded explanation helps!
SOURCE: carburetor adjustments yamaha 450yfz 2004
Turn both the air screw and throttle screw inward until they LIGHTLY seat. Now back each one outward one and one half turns. This is the factory setting. I am going to give you a web site where you can download a free PDF factory service manual for a YZ450FR. I believe the model is a bit different but the carbs are the same.
Go to this site and you can download a free PDF service manual. The site may take up to 25 seconds to fully load the list of manuals. It is worth the wait. www.carlsalter.com/motorcycle-manuals.asp The service procedures are there in detail and a lot of diagrams and photos. Don't mess with the pilot screw unless you have to, Part #11 in the diagram.
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