Starts and runs like **** then dies

Ok I assume what you are trying to say is that once it starts with the choke on and you turn the choke off it dies.
This behavior usually means that the engine is leaning out. This means that it is not getting enough gas. Often if you put it back to choke fast enough it will keep running though very roughly.
The problem is the main metering jets are plugged or there is some kind of obstruction in the carburetor. This is assuming of course that the tank is full of gas and the fuel filter is clean.

first check for spark when it dies then check to see if the needle valve seal swelled up from our current fuel

Check fuel pump delivery for adequate flow. If when choke enrichment is applied and it runs, then dies when choke is off, fuel flow is inadequate and needs choke enrichment to compensate. Also, if engine dies or chokes when increasing rpms, this will be a symptom of too little fuel. Check fuel flow at pump, then, if flow is good, check for carb blockage, unscrew the jet and inspect, check the operation of the solenoid at the bottom of the bowl, it has a wire attached to it. Trace that wire to where it plugs in, unplug it and touch it to the positive pole of the battery, it should retract. If not you need a new solenoid that has the job of metering in gas to the inner bowl below the jet assembly.

Hi and welcome to FixYa, I am Kelly.

The symptoms you describe are typical of debris in the fuel bowl of the carb. I am going to assume for the moment that it is a similar to a Briggs and Stratton engine. Engine model type and serial information is what we work with to resolve engine problems regardless of the mower brand. I note you said it was a Scott Bonner SX4000 engine. I am also assuming that you have the 18 inch GreenKeeper like this one:
http://www.rovermowers.co.nz/www/rover/Home.nsf/13a851504c93cb754a257821000cfc6b/$FILE/28045.pdf

I do not have access to Scott Bonner Engine parts or technical data but if you clean out the fuel float bowl and it is still hard to start then the choke function of the engine is not working. Your engine has a "Bowl drain" screw on the bottom of the carb float bowl for removing carb debris or water in addition to a bowl nut.

Most likely due to the throttle cable needing and adjustment so that the choke is closed when choke is selected. Adjustment of the throttle cable is as simple as losening the clamp screw (Do NOT remove it) on the side of the carb throttle control and moving the throttle to STOP then re-tightening the clamp retaining screw. This should allow your choke to move full closed when selected. You may have to experiment with full throttle and STOP throttle control positions with the clamp screw loose but not removed to see which position will give you full throttle, choke and STOP. After each adjustment you MUST tighten the clamp screw to test the throttle cable travel on the the throttle plate. Beleive it or not... the person that assemble the mower made the intial throttle adjustment. It may have never been tested before you got the mower.

If you still have problems.... this is the brief way to start an engine that has choke problems. Remove the spark plug and squirt a splash of gas into the spark plug hole and then re-install the spark plug. Just don't over-do the splash of gas and if your messy wipe off the excess so you do not start a fire. (This method always works but you occasionally foul a spark plug with too much gas)

Thanks for choosing FixYa,
Kelly

Check your magneto. I have had them cease to function whenever they would get hot. When cooled down, they would work.

the choke should be in the off position when running. YOU put the choke on to start it then when it starts u turn it off. If the choke is on the engine will die when reving

replace the spark plug first then go from there, try starting fluid if flooded but also as soon as it fires during starting pull back the throttle off choke sometimes you don't want to give the unit full throttle when starting 3/4 throttle is good enough

purchased new unit

If the choke has an adjustment point, then adjust the choke to a good overall working powerband. If it still doesn't act correctly, look for vacuum leaks. If it is an option, you may want to install a manually operated choke lever.