85 hp mercury starts right up but dies when you let off the key

by Jeff
(muskogee,oklahoma)

I took my boat out on Saturday it ran great i started it 10 or 15 times threw out the day when i put it in the water Sunday it wouldnt stay running, it would bust right off everytime but as soon as i let off the key it would die as long as you keep cranking it will stay running but i didnt want to burn the starter just to go fishing.. it is a 1979 mercury 85 hp motor thunderbolt electronics its been a really good motor can you please help...thanks in advance..