1999 Force 40 hp by mercury

My 1999 Force 40 hp runs well, when I power down and drop motor out of gear, it idles down and shuts off,
sometimes with a backfire. any suggestions?

Could be sucking air through one of many gaskets or seals.

While in gear/with some throttle it may appear to be running fine because it's getting enough gas. When you return to idle and no load too much air/not enough gas is causing it to die out. A backfire or "lean sneeze" would normally be present.

I would check the powerhead base gasket, carb gasket(s) - where they are bolted to the intake, transfer cover gasket (if present on this model) adjacent to the cylinder intake ports and the head gasket (this is a Chrysler style block, not a Merc block so it has a head).

Check under the flywheel and/or in the lower cowl for an oily/goopy mess (caused when fuel/oil/air gets blown out of a crank seal - the gas evaporates and leaves the oily residue behind adjacent to the toasted seal).

Basically look anywhere you think air could be getting into the crankcase without having to pass through the carb...