I inherited my father in laws 77 model boat and motor. Most of the time it starts OK but when I put it in gear to go forward it dies. It will finally go but it takes several trys to get it going. What could be the problem and how to fix it. Thanks

Idle too low? Sounds like a fuel problem.

If that is a 77 model 150, it will be an inline 6. I would take the carbs off and do a clean and vat job on them. Then set the air/fuel mixture screws at 1 5/8 to 1 3/4 out.

dies while idling in gear, or dies when you put the hammer down?

regardless, if it's been sitting idle for more than a few months a carb rebuild is in order anyway. be sure to replace float valves and seats. also water pump impeller. change gear oil. drain & clean the fuel tank, get some fresh fuel in there. make sure the tank vent is clear. new spark plugs

but before ya do anything else...compression test - no point spending time & money on a motor with bad compression.

next get a service manual - I highly recommend the factory shop manual - will more than pay for itself with your first carb job.

Where can I get a service manual-and will it show me how to clean and take off the carbs. Thanks for all the replys

Any Mercury dealer will have them.