Starts but will not stay running

Thank you so much Kasekenny1 for your reply. I didn't mention that the engine was just rebuilt, And ran well enough that I was able to break in the Cam. The starter would not disengage and I had to pull the wire on it to stop it. So I changed the Ignition switch thinking that was the problem. It turned out the flywheel needed to be re ringed and the starter needed to be rebuilt. So when I put it all back together it would start and Rev in start position, then turn off immediately when I let go the key to the run position. The carburetor is also newly rebuilt its getting fuel but I am sure there's vacuum makes to deal with. It's the I post on the starter solenoid that wasn't getting any juice. Thank you so much again, getting help with things you don't understand is such a blessing! That is a great write up!