The fact that the plug shows a "whitish" color, AND you have intermittent "popping back through the intake", AND that you can "somewhat smooth out the running" by partially closing the choke, all confirm that there is not enough fuel reaching the engine. The plungers do sometimes get gummed up and reluctant to open fully (and sometimes, not at all). You can try removing the fuel solenoid and soaking the plunger tip in a small container of spray carb cleaner. Fill the cap from the can with cleaner and stand the solenoid on its tip in the cleaner filled cap. Prop it up so it can't fall over. Soak it like this for a few minutes then push the plunger into the solenoid body a few times. The plunger should have a slight resistance or spring pressure when pushing it in. It should pop back out when you quit pushing it in. Repeat the soaking and manual moving of the plunger a few times until it operates freely. You can "bench test" the solenoid by using a 12VDC source (the tractor battery will do) by connecting the body of the solenoid to the negative pole of power source and the wire lead of the solenoid to the positive pole of the 12VDC source. The solenoid plunger should retract into the solenoid body when 12VDC is applied, and pop out when the current is taken away. You might have to push in ever so slightly to get the plunger to retract because in some cases, the plunger will be sticking further out of the body with the solenoid removed from the carb than when it is installed. This is because the design calls for the spring to actually PUSH the plunger valve against its seat with a bit of spring pressure. If the plunger does not "jump into" the solenoid body as soon as you apply 12VDC to it, just give the plunger a little nudge to get the plunger close enough to the electromagnetic field for it to "grab hold of" the plunger. The plunger should remain retracted for as long as the 12VDC is applied. While you are into this carb, check to make sure that the main jet is not partially clogged or varnished up. It is also essential that you have a good, free flowing air filter in place as well. While in this case it appears that your engine is "starving for fuel", you must remember that the fuel is "drawn out of the bowl by venturi effect". If you do not have good FLOW (which equates to induction air velocity) through the air filter, the venturi effect will be weak and so will the volume of fuel. Another thing you want to make certain that the cooling shrouding and fins have not been converted to a mouse haven over winter. These critters will carry all sorts of stuff in to create their condo. This can completely block the air flow from the fan. Overheating will also create a "lean burn" condition (just before everything siezes up).

***"It appeared to seize up"*** Did this "seizure" occur while the engine was running? (I was driving along and suddenly the engine quit and it appeared to be seized up.) Or did you try to start it up cold? (I was going to use the tractor but when I tried to start it, it wouldn't start and it appeared to be seized up). ***"I can see the starter popup and turn the shaft but the motor will not start - just see the shaft spinning."*** Does the starter actually "spin"? or does it just "rotate" the shaft maybe half a turn and stop? Please get the ENGINE numbers from the engine, post the Model number & Type number.

Check to see if the vent on the fuel tank filler cap is clogged. This has the classic symptoms of that. You can try running the tractor with the cap loose which will allow air to vent past the threads as a way to check if this is the problem. You can clean the vent holes with a small wire or unfolded paper clip or blow it out with compressed air.

Remove the deck so you have a good unobstructed view of the ground drive belt system. Have a helper sit in the seat and push down on the clutch/brake pedal while you examine all the linkages in the mechanism from below the frame. You are looking for worn or loose pivots, pins or anything that can limit the travel of the components that put enough slack in the ground drive belt for the transmission input pulley and shaft to become "unpowered" so you can shift. Have you also noticed the clutch engaging at a lower pedal position when you try to get moving from a stop? That can also be a symptom of the same issue. If you have recently changed the belt and did not use an OEM spec belt there is a chance the belt is a fraction too short. Basically, something is causing a bit of power to be applied to the belt when the clutch is pushed down. Here is a link that might be useful: 917.272751 OM