Re: 1975 70hp Evinrude, wont stay running, please help!
First off, you need to get yourself an OEM service manual for your motor, and learn a little bit about it including the terminology. You don't have 3 starter solenoids on the motor. You have 3 ignition coils, which I suspect is what you were referring to.
Start with the basics; compression, fuel, spark. Compression needs to be reasonable reading (+100 PSI) but more importantly all cylinders need to be within 10% of each other. Check for spark jumping a 7/16" air gap with a good snap and a bright blue bolt. Check your fuel connections and fuel lines for any air leaks, make sure your fuel pump is pumping fuel to the carbs by disconnecting the lines to the carburetor and making sure that its moving fuel when you crank the engine.
Now, is your problem that the motor ran for 30 min and then just won't start up again, or is it that it can consistently run for about 30 minutes before stopping, and then you can't restart? If you can get it to start again once the motor cools off, you are probably not looking at a fuel or compression problem. 30 min is longer than it would take to drain the fuel from the carb bowls, so your fuel pump should be working. You still want to do a compression test, as you can start and run that motor on two cylinders.
If you can get it to consistently start, but it dies after a time period longer than 5 min and you can't restart for a while, I would suspect an ignition problem. As the motor warms up, metal expands and can cause electrical shorts in the motor that will cause it to immediately die. Once the motor cools down, metal contracts again and it will be able to start. Start your motor and wait until it dies, then test for spark. If its not sparking, you need to troubleshoot your ignition system.
Rgds