My 2005 Grizzly 660 stalls imediately upon giving it gas. I had a new needle and seat installed due to the needle sticking. The carb was cleaned thoroughly and everything installed back to spec. It starts, idles nice but stalls upon throttle up.

There isn't one. You have to put one in-
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Grizzly 660 Shifting problem. - Yamaha Grizzly ATV Forum

www.grizzlycentral.com > ... > Grizzly Engine & TransmissionMar 12, 2012 - 4 posts This is a discussion on Grizzly 660 Shifting problem. within the Grizzly Engine ....2005 grizzly 660 gear position sensor problem.

sounds like the carb needs cleaning as float could be stuck !?

1. remove the hose from the carb and see if the petcock is shutting off the gas and if not clean, repair or replace the petcock.
2. look in the tank and see if it has rust in the tank even a small haze of rust as rust will cause the needle and seat to stick in the carb and make the gas leak. If the gas tank has a lot of rust it may need a tank rubber liner kit added or even a new tank to stop the leaking.
3. if the bike has been sitting for a while it may have trash in the fuel system(carb/petcock/tank). Rust or any trash can cause a leak. Properly clean out the carb and fuel system and add a new filter when done.
4. Improperly adjusted or bad needle/seat or float in the carb will cause continual leaking. Replace the needle/seat/float and adjust the new one when installed. Check your service manual for float adjustments.

Clean the carb again and add some fuel/carb cleaner.

CLean carb take fuel off and clean needle is stuck in carb might need carb kit mopar Kenny

you cant connect all the wires, 2 let it start and 2 kill the ignition.

sounds like the floats in the carbs are sticking.....need cleaning. as maybe some muck in them. or if been stood over winter they gum up!!.....i would give the carbs a good clean & strip down !st...

You do have a sticking float. You can test it with the bowl off by moving the float up to shut off the fuel, and down to allow fuel to flow. Look closely at the float needle seat to make sure they aren't damaged.

MAke sure your tank vent is clear.If so you need to pull the carbs and clean the passages and jets.You may have a stuck float or needle