Starts but will not stay running

If this is stalling as soon as the RPM comes back down, I am pretty confident that you have a vacuum leak or a fuel/ignition issue. So let's start with a vacuum issue. Here is a guide that will help:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Then let's check a fuel issue by using starter fluid and see if you can keep the engine alive by spraying it in the carburetor. If you can then we may need to adjust or rebuild the carburetor.

Let's start with this and go from there. Thanks

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!

Okay. That helps. However, can you get a video of what it is doing now when it starts? Seeing and hearing it will really help to ensure we are on the same page. Just take it with your phone and post it below.