let a buddy borrow my boat, i get it back and take it out, starts up fine can rev up with lever but go to put in gear just dies. Smoke rolls out when i rev it up like maybe to much oil. Think new plugs and a little more gas will work? its a old merc inline 6 cyl 115

Could be the fuel pump diaphrams are getting weak.

If it starts up, revs up, and then you put it in gear it dies.  I would look into possibly something gone wrong in the gearing/lower unit which is causing the motor to stall out.  If it was a fuel issue it wouldn't start nor would it rev up out of gear.  There's something more wrong with it then a fuel issue. Might want to question your buddy on what he did when he used it.

My motor did that after the oil injection quit. The cylinders got scored and the engine lost compression. (at least that's what the repair shop told me) Just would not go with a load on the motor.

My first guess would be the carbs.  Low speed jets are too lean or dirty passages in the carb.  I thing those carbs have a small hose that goes across them for the low speed fuel, make sure not of those have come off.  If opening the low speed some doesn't help, might want to see about cleaning them

I had similar issue with an old motor, and it turned out to be the carb. jets just like way2slow mentioned. Give it a good spray/ or soak and see if   it solves the problem. 

I had a similar problem with my 2004 Merc 90 HP three cylinder. Just like the two guys above mentioned, it was fixed by resetting the lean jet screws.

If you don't already have a good repair book, I picked up a Mercury outboard repair book from Seloc on Amazon for around $30. I've used it to diagnose problems, rebuild my carbs, rebuild my fuel pump, resync the carbs, etc. It has helped me save hundreds from not taking my boat to a mechanic for every issue I've run into.

Good luck.

Link to book: https://www.amazon.com/gp/product/0893300675/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

Listen to fellas talkin carbs. Run some seafoam through it