Engine will not run following a brief shutdown.

[Snowboy150]

I have a 1991 Johnson 70HP, on 1991 Forester 16 boat that I have just purchased.
The boat and motor run well the only problem I have is that it is difficult to start after the boat has run and stopped, for a period of time. The time can vary from 20 min to an hour.
I have tried chock, no chock.
What happens is that the motor starts but when I go to put it in gear, either forward or reverse, the engine stalled, almost like there is not enough revs.
Can anyone help?
Thanks
Geoff

 

[metriccrescentwrench]

Start out with the basics. Check the compression on each cylinder. What do the spark plugs look like (if they haven't been replaced in quite awhile that could be your problem)? Check that you have a good energetic spark with a spark tester.