I have a 2000 Mitsubishi Montero Sport 3.5L 6cy. I just changed all the belts and put new tensioner pulley in, why am I smelling burning rubber?

 

I had this exact problem, it turned out I had accidentally tightened the belts too much. Assuming there is no squealing noises, tension on the belts is likely too tight.

EDIT:
mbwall45 made me think, there doesn't necessarily have to be a squeal for it to be loose.

The way I was taught to check the tension is by finding the longest run of the belt between two pullies and checking the deflection(or slack). Place your finger underneath the belt, right in the middle of the longest run, and lift up on it, the belt shouldn't move more than 3/4" but you also don't want it so tight that the belt hardly moves. Also check your tensioner pullies, if one of them is not tightened down flush with the tensioner body it will put the pulley at an angle which can lead to both the pulley getting worn out prematurely(or falling off because the nut isn't set right and may work itself loose) and the belt having improper tension/angle on it.

 

Might be because belt is made of rubber? which you said is new installed Would like to check if the tensioner / serpentine belt isn't tight enough? Is there a minimal smoke?

 

Good advice on belt tension. You don't want to overtighten the belts. Rubber is one of those funky materials that tend to shrink when it warms up. So, if you start with overtightened belt - it will be even tighter once it gets hot. One other thing to check is pulley alignment. If they are misalligned, belt will flex more, rub (smell) and wear out faster

 

Older Mitsubishi Sport engines had several V-belts.