I just bought a 85 150xr2. It's been sitting for quite a while I think. I know this is a broad question but where would be the best place to start as it is not firing on any cylinder.
You can view a parts diagram and parts list for your particular motor on the Mercury Express Parts or Doug Russell Marine web site. Of course, a better thing to do is get the OEM Mercury shop manual for your motor. You will need it to do most any maintenance or repair on the motor. Tinking with that 150hp beast without a shop manual for step by step guidance is asking for trouble in my opinion.