I would say the tuners were giving you decent advice. 39% on the front is on the light side. I’d try to get that up a few points. TJ is right, a guy your size should have the opposite problem. Your kart should be over flexing. It’ll be interested to see how the IKart handles someone your size. You may need to try several different seats to get where you want to be. In addition to being too far back, perhaps your seat is too high. You should have it as low as possible, with the bottom somewhere below the frame rails, maybe a couple cm or so.
I just googled Twister seat and that thing looks super stiff. It looks like a flat, or oval kart type seat, which isn’t what you’re going to want in a Yamaha sprint kart. Most oval kart seats don’t really need to flex, as they aren’t really integral in the chassis function. They’re more a place for the driver to sit and keep his/her weight down low. The Tillet T8 is going to be a way better application for the Ikart or the Margay, for that matter. If that doesn’t work for you, contact me and we’ll see if a DeepSeat puts the weight were you need it, yet still keeps you comfortable.
One big thing, is to make sure the seat isn’t in a bind when you install it. It should sit neutral in the kart when you tighten up the bolts. Make sure you bend all the tabs so they are parallel with the seat surface, and fill any gaps with spacers or washers before tightening. You definitely don’t want the seat to be pulled in any direction when you tightening up bolts.
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