just a thought but, experience with another type of caster type setup would tell me that there is something about the angle is bad, so something is bent or not manufactured correctly or just worn out, do the new ones have some adjustment? I have foam filled front tires on my 1998 model scag, no issues.....?????
If possible, remove the wheel assemblies and bend the upper fork so that the centerline of the axle is behind the centerline of the pivot pin. This increases the caster so they will track better and actually act like a caster should. I used to align front ends of cars and trucks. This is how you increase the caster effect and that should stop the wobble. Hard to explain in words and a graphic picture would be better.
I'm starting to think the front left part of the frame that the caster arm bolts to is bent or twisted up slightly. I replaced the arm thinking it was bad since it was cracked and welded back when I bought it. Since putting on the new arm it still seems as if there is less weight on the left front tire; I can still push that caster wheel around with my foot. Everything has been replaced on the front, even new set of front caster wheels and tires.
Den052,, I understand what you are saying but I'm afraid if I mess with the caster angles like that it may affect my deck angle too. Plus like mentioned above, I think it's just in the left side throwing everything off. I'm probably going to just suffer through it for the rest of the mowing season then this fall I'll take it to Scag dealer and have them look at it.