My 71 turns on immediately, but when I shift into gear it loses power and eventually turns off. Cutlass Supreme 455 Mondelo motor. What could it be? Spark Plugs?

 

When was the last time it was tuned up?

 

recently, but not sure it was done righ

 

Recently, but not sure it was done right. The mechanic thinks that less powerful plugs were used

 

This should be a BIG clue that he knows nothing. Find someone that knows something.

 

Without knowing the tune up settings used and specific symptoms no one will be able to point you in the right direction on the interwebs.

 

The car turns on well. Sounds good, but once I put the car on reverse to get out of the garage, the car starts to sputter, then turns off. Now the car experiences a hard time to start. I finally get it to start and I rev up the engine, and then again when I shift to reverse or drive, it slowly turns off I would like to find a good mechanic, but having problems with that.

 

It's a requirement you own the 1971 OEM Oldsmobile Chassis Service Manual (CSM) which 'specifically' details the service tune-up specifications for your engine. Purchase one ASAP & next time you find a mechanic, you provide the mechanic with at least the pages in the 1971 CSM which specify the tune-up specifications. The CSM is your 'bible' for your vehicle. Purchase a used paperback book, CDs/DVDs miss pages, poorly copied, unreadable, and lack detail. Additionally, an original OEM 1971 CSM contains the color wiring diagram. You need the CSM - buy it. Do you happen to have a tune-up sticker/tag located on your radiator cover plate/shroud which demonstrates tune-up specifications for your 455 cid engine? Does your car have A/C? This could be as simple as adjustments to the correct RPM setting(s) when in gear vs. in park/neutral. It could be a minor vacuum leak or a vacuum hose disconnected. It could be a minor choke adjustment. Whatever the case, you're throwing darts unless you have the 1971 Oldsmobile CSM which specifies the tune-up specifications for your engine.

There may be other manuals out there on eBay - some in better/worse condition some w/ or w/o shipping - do your research but order one ASAP so you have a leg to stand on and a card in the game.

Original 1971 Oldsmobile Chassis Service Manual

 

This is representative image of a basic tune-up decal/label/sticker/tag (Vehicle Emission Control Information) normally located on the radiator top cover plate/shroud - it provides the basics. NOTE: This decal is for a 350 cid.

 

I doubt very seriously that the decal is going to apply to his Mondello Engine

 

^^^^^^^
Check for vacuum leaks as well as basic tune up specs.