After 1/4 throttle it spits and sputters. I changed internal fuel pump and spark plugs.

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..

Could be a weak spark, since a weak spark will break down under load conditions. Pull the spark plug and make sure their is a fat purple spark. It could also be an air leak between the carb and the intake manifold causing a very weak gas to air ratio, running too lean.

Fuel starvation (supply or wrong carb or tune) and/or insufficient or poor quality spark.

Make sure the air filter is clean and only test with it connected as it sometimes makes a lot of difference. It could be worth checking the exhaust for blockages.

Pull plug hook it to wire and test for spark...place against bare metal and turn motor over,look at plug if u see spark ?? your good there if not try new plug could just be fouled., Then work your way through fuel system....

tank airiation fuel tap filter tankfilter clean? sludge removedl a canister automotive engine cleaner for the fuel are by cleaning carburetor( if there is a L+R re-adjusted) are the gaskets changed and correct placed especially between cilinder and carburetor. there can be especially a small channel in it that it was wrong replaced or is the compression good enough ?
Compression is the very most important thing there is to look at the exhaust site to the piston that must be smooth otherwise there was a problem by overheating piston if there are grooves there can be a wrong adjusting of the carburetor or by a quality problem of the used gas

You need three things to make any gas engine run, in this order double check make sure you have proper timing set, spark and fuel. Could be bad gas or a bad ground from motor to frame.

sounds like you for a fuel problem like floosing crankcase with fuel, is it FI or carb

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The problem would appear to either be with the mixture or with the head. Too rich a mixture (too much fuel) will cause black plugs - when you take them out do the engine and plugs smell of petrol? You need to adjust the carburettor. Mark where the throttle screw is to start with and then slowly screw it (in - I think) to reduce the petrol flow.
The other alternative is that their is a problem in the head. Does the engine blow a lot of blue/black smoke out when it is hot. If yes it means a full engine strip down!

sounds like an air leak

Adjust the fuel idle and the accelerator cable.