1999 Mitsubishi Eclipse timing belt tensioner/pulley isn't

Engine Mechanical problem
1999 Mitsubishi Eclipse 4 cyl Front Wheel Drive Manual 125xxx miles

i am doing a timing belt repair on my 99 eclipse 2.0 non turbo. It is a 5 speed manaul with appox. 125,500 on it. I replaced the water pump as well. When I put the timing belt on and pulled on the tensioner to make it tight, it will tighten for a second then loosen back up. There is about an inch of play in the belt, but only in the area where the tensioner is. By that I mean that around the cams, crankshaft, and water pump it is still tight. Is that too much play for the belt? Also is there anyway to lock the tensioner into place? I have a paper that says to use an 8mm allen wrench to lock it, but I cant find out where to place it. Thanks, Jason