I'm not sure about the weight. Conventional wisdom says the lighter the better, because grip is more or less the same while acceleration should be better for you. The fact that your friend overtook you on straights says to me that he had a better engine.
Now, allow me to give you a long explanation (as usual) on what I know about moving your body in a kart.
A kart seems simpler than a car. Well, I'm not sure, because the whole idea of a kart is that it has no differential and no suspension.
You know the zen approach I favor: be the kart.
Now, imagine you do not have differential and nature has provided you with a rear solid axle. Just like an radio controlled car, btw.
You are sitting peacefully in the track, when your driver starts to move and turn your little wheels. You have to "scrub" the inner rear wheel, because you have a solid rear axle, no differential. Both rear wheels, rolling the same distances at the same time, will try to keep you going straight.
So, ideally, to turn around a corner, your driver needs to "lift" your rear inner wheel, converting you, a four-wheel kart, into a tricycle for a few moments.
Martin Lyell shows how it's done
When you drive, the first idea is to "shake the rear bridge" (shake your groovy thing...
) by moving a little your body towards the outside of the curve.
Notice you do it naturally. Why? Because the steering wheel has an angle with the vertical, when you look at it sideways. When you move the steering wheel your outer arm stretches and pulls your outer shoulder toward the front of the kart. The degree of movement depends on the radius of the curve: the tighter the curve, the less you need to move your body to lift the wheel.
Watch Lyell's shoulders: the guy has moved himself towards the outside of the curve, to take some weight from the lifted wheel.
I confess I even exhale when I turn and I inhale when straightening the steering wheel, like when you do weight lifting, but that's, perhaps, taking the idea to the extreme... In any case, for me, it works.
If you cannot lift the inner rear wheel enough by moving your body around, you can change the caster angle, increasing it. The followin drawing, in Word, conveys this idea, even if it's not very well drawn:
When you turn the tyre towards the inside of the kart (front circle) the wheel is "lifted" from the ground: this causes this corner to "go down", lifting the opposite rear wheel
You can also increase the Ackerman setting to achieve a similar effect: that is, you make your inner wheel to turn more than the outer, increasing the lift of the rear inner wheel, making slightly easier for you to "tricycle" around the corner.
I have a few moments (I hope you have them too...
) to explain something about what happens when you succesfully lift the rear inner wheel:
Think about this for a moment: braking while cornering on a kart causes massive understeer. The kart continues in a straight line. Why?
It's not logical, as Spock would say. You're transferring weight toward the front of the car by braking, aren't you?
Besides, when you accelerate, the kart improves the turning. Again, why?
In a regular car you find the opposite: acceleration causes understeer, because the weight of the car transfers to the rear wheels, don't they?
Notice this: when you lift one rear wheel, all the accelerating or braking force in the rear axle is provided by the outer rear wheel
. So, when you have lifted the rear inner wheel, any braking or accelerating force of the rear outer wheel, which is out of alignment with your Center of Gravity (CG) causes a torque that makes you, the kart, to turn around your central axis.
Thus, the understeering caused by transferring weight to the back of the cart while accelerating is dwarfed by the torque of the outer rear wheel.
If you followed me up to this moment maybe you're a little confused. Perhaps this image will help:
When you're on three wheels you oversteer by accelerating (green arrows). The rear inner wheel (lower left square) is up in the air, thus exerts no force.
So, if you brake in the middle of a curve, while "tricycling", you "torque" yourself towards the outside of the curve and you oversteer severely. If you're at the limit, this will "break" the front tyres, making them to slip. Once you've blocked the wheels, the friction go down (the dynamic friction factor is smaller than the static) and you cannot recover until you stop braking and regain your "foothold".