When the tires hook, the load IS greater. If you don't understand physics, I don't know what to say. Traction is needed to get out of mud (don't the tires spin in mud ? ) You're all over the place with your analogy, you don't add an external variable to the equation. And again, secondaries (and primaries for that matter) control AIR, not fuel. If you don't understand how a carburetor works, I don't know what to say. I could get into technicalities, involving piston speed, rpm, air velocity, fuel mass, load force, inertia, etc. I understand quite well the physics of carburetor operation, but I'd rather not divulge my qualifications on this matter.