Originally Posted by thermite
I actually think it's the valve side which is the heaviest on my wheels.
Hmmm, well color me confused. This is just a stab in the dark but maybe it's really your MTB tire that's imbalanced, as someone else suggested. You could deflate and rotate the tire on the rim to figure out if it's that or not. Then try taking the tire completely off the wheel and leaving the tube on there partially inflated and see what you end up with. I'd be interested to know.