1996 Mariner 9.9 hp four stroke dies when shifting into gear

I inherited a 1996 Mariner 9.9 hp four stroke. I can start the motor but it will die a short time later. It will die when shifting into gear or raising the high speed idle lever. What is the best way to diagnose the problem?

If it has been sitting for a while there are a couple of things I would typically address.

The stalling may be a result of a gummed up carb. If it wasn't drained and properly (winterized) before being laid up it wouldn't be uncommon for it to be "full of cooties".

A partially clogged carb may allow the motor to run in neutral, on a flusher or in a pail of water but will bog out or stall out when put in gear (when the motor starts demanding more fuel).

So a complete carb tear down, thorough cleaning (not a "magic in a can" solution) is in order - and replace/rebuild any suspect parts (sad looking gaskets, needle valve etc).

Second, at minimum I would change the impeller (water pump). Depending on the type of climate the motor was (living in) they might last a couple years or they may last a few years. Sitting baking in the sun kills rubber quick, so if it was "parked" outside it may be shot. It's cheap insurance to ensure you don't overheat the motor and kill it.

Of course, check the plugs and replace if they look (old). Check all the wires and hoses - make sure nothing has chewed on them, nothing is rotted etc.

And if you are going to pick at it yourself, get at least a Seloc's manual. It will walk you through most general maintenance items and tell what to do and just as importantly, what "not" to do when you are working on it.