I have a 1977 Johnson 75 Stinger.  It cranks and runs great. If I stop for 30 minutes to an hour to fish or troll, when I get ready to crank up again the engine has lost its prime.  I have to pump the bulb about 3 times and it cranks right up.  Why am I losing the prime?  The boat has a 20 gal. built in tank. I put a new fuel line and filter on but did not help. I have now installed a anti-syphon valve at the tank. I've not had a chance to see if that helps.  

Has anyone had a problem like this?

Tough to diagnose via the internet, but I would check for dried/cracked fuel lines or poor fuel line clamps . Sounds like you could be sucking air somewhere.

Another thing to consider - Any hose that carries fluid can deteriorate from the inside and still look good on the outside.Older rubber fuel system components don't stand up well to todays gasoline blends. Considering the age of your engine, be thorough when inspecting the fuel system.

If the motor runs fine at WOT and idles fine, I'm not too sure it's loosing prime.  There are some checks I would make the next time on the lake.  After making you first run and letting it sit to where you say you have to pump the bulb, before trying to start it, screw the plugs out of the bottom of the fuel bowls and see if plenty of gas drains out of them, if it does, it's not loosing prime.  It's very common to be able to squize the bulb after shutting an engine off.  If very little or no gas drains out, then I would look at rebuilding the fuel pump and carbs and set the float levels.

I would do a link and sinc and check the choke is fully closing the butterfly when you press the key/choke button.   Dirty carbs can also make one very hard to start and require manually priming everytime.

Another thing that can cause one to be hard starting is a weak battery or dragging starter.  Yes, it's cranking over, but if cranking slow it will not generate a hot enough spark and it takes more fuel in the chambers to start it.  The CDI ignition system on that motor requires it be spinning approx 250 rpms or more to generate a full spark.  

One more thing.  If I run at a very slow speed or idle, the engine will eventually die.  

Thanks again for the help. I appreciate your knowledge.

Then you might have a needle leaking by the seat.  This causes the carb to load the motor, smoke like a freight train and idles rougher and rougher until it dies.  Rebuilding the carbs and making sure the float levels are set correctly when done should take care of that.