Fuel system completely flushed carburator completely cleaned, new gas

Based on the information you have provided, there are a few possible causes for the issues with your 2011 Arctic Cat 550 EFI. Here are some suggestions for troubleshooting:

Check the fuel injectors: If the fuel injectors are clogged or dirty, it can cause issues with starting and idling. You may need to clean or replace the fuel injectors.

Check the fuel pressure regulator: If the fuel pressure regulator is not functioning properly, it can cause issues with fuel delivery to the engine. You may need to replace the fuel pressure regulator.

Check the air intake and filter: If the air intake or filter is dirty or clogged, it can cause issues with fuel delivery and combustion. You may need to clean or replace the air filter.

Check the throttle body: If the throttle body is dirty or clogged, it can cause issues with fuel delivery and combustion. You may need to clean the throttle body.

Check the ignition system: If the ignition system is faulty, it can cause issues with starting and idling. You may need to replace the spark plugs or coil.

Check the battery: If the battery is weak or not providing sufficient power, it can cause issues with starting and idling. You may need to replace the battery.

It's important to note that these are just suggestions based on the information you have provided..

Have you checked the Crankcase seals? I have 2 Big Bosses with 250's. 1989 + 1992

Both were terrible at starting and idling....after much troubleshooting I finally changed the crankshaft seals. (Which btw looked fine from the outside!) machines can be Cold now and will start 2 or 3rd pull with the cord now. Idle perfectly.

Also the oil injection system stopped working intermittently. Machine would just quit.....crank and crank it forever. So I just started using 32/1 mix and unhooked the Oil Injection cable from the Throttle.

Sounds like the carb may not be completely clean...Did you get the GENUINE OEM carb??. When someone tell's me they replaced the carb (why?) .. I have been repairing motorcycles and ATV's for 45 years. only replaced 3 carbs in that time frame..ALL the others I was able to clean and repair...- I get the feeling someone is guessing at how to fix an underlying problem?? Did you use all new OEM gaskets? Did you remove the piston to clean it ( bead blast the ring lands?).. is the piston on in the correct direction??. rings on correctly? .. hone the cylinder?? .. check the valves for leakage?.. All bolts/nuts properly torqued?, ..OK.. as you can see there are many variables that need addressing before any kid of accurate assessment can be made!! My suggestion is to find a competent factory trained mechanic and pay him for his service!!

was just working on a scooter with the same prob, turned out to be the cdi. I'd check other things before replacing that though. cracks in the intake boot, adjust valves, test plug and coil.

backfiring is usually a sign of an air leak in the intake manifold

Harry, dirty carb or injectors.
Check the attached links,Manuals and guides, Good luck with your Bike
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ATV TROUBLESHOOTING AND MAINTENANCE
ATV Service and Repair Manuals
http://www.carlsalter.com/motorcycle-manuals.asp

U have a fuel delivery problem. Carb float not getting stuck? Carb jets adjusted properly?

Its either needs the fuel system, including professional carb cleaning, or there is a ignition system problem.
Its probably the carb!!!!!

If it's been sitting around for a while it could be the idle jet has a little craip in it (it don't take much) but if it's spitting fuel out then I would make sure the float levels aren't sticking.

Adjust the fuel idle and the accelerator cable.