https://imgur.com/9WKxgZn
Problem: I lived in NY so 8 months of the year the bike was in the garage covered up. only has 9k miles on it, driven very lightly.
My 09 Ninja 250r Idles great when the choke is on. When i press the accelerator 10-15% the rpms drop until the bike stalls. At first i thought it was a dirty carb so i disassembled it several times cleaned and put them back in. Same issue.
FIX:The circled hose was ever so slightly stretched out and would not fit snug, even with better hose clamps on it. I swapped out the hose with a 1mm smaller one to guarantee it fits and put on new hose clamps for insurance and it fixed my problem.
The bike is a little jumpy at low speeds, but on Highway speed it runs perfectly. i suspect a minor tuning is in order