having 2 10 inch tires on the rear doesnt help much either, he isnt that much over weight, that might be true with the soft sprint chassisWhen you're way over weight the frame and tires mush out and don't work right. There's no widow of adjustments you can hit to make it perfect but you can slightly exaggerate all the settings that loosen the rear and I wouldn't run less than 12 psi in any tire.
My mychron data shows 88MPH without corner speed. I have a 31HP stock appearing motor.who told you, that you were doing 90?
What engine?
Hard to turn was probably because you had the tires on wrong.
wide in the back, kart wants to go straight, its not a hot rod.