You're gonna need some movement in those rear springs as well. With both ends solidly mounted there will be no room for the springs to move as they flex. You need some shackles.
Why aren't the tires made from a more rubber like material? Some old Revell kits had rubber like tires. Alan Rabb of Ma's Resin uses a rubber like material.
At a loss for some adjectives , then reading some of the other post I found just the right term, FINE WORK. The front wheels really make it happen. Fine Work
get out yer silver and gold and start mixin'. I find it's a trial and error kinda thing. Do some home work and Google Carburetors to find the color of the one you're working on. Good luck.
That's the problem, TOO much speculation from people that don't know and not enough from those that do know but wont tell ,insider information,trade secret, secret club, etc. etc.