Only runs 40 Degrees advanced

Garrett Ellison said
It wouldn’t be a rich mixture that would cause this issue, it would be a very lean mixture. Much like at cruise rpm when the cylinder charge gets diluted by exhaust and the carburetor is just off idle and it eats up to 52 degrees of mechanical and vacuum advance. But, if it were this lean, it would have a tendency to overheat. It sounds more like a vacuum leak or more likely that changing the sniper to not control the distributor has caused an unseen change in the fuel delivery curve. I would like to hear more specifics of the build, though. Displacement, camshaft type, static compression, etc.

Engine specifics were in earlier post but it is a 360 I rebuilt years ago it’s always been just a cruise in car so it doesn’t have that many miles. It has a whiplash Cam flat tappet MSD pro billet distributor at the moment with six AL box and matching coil has been running really good until this distributor got jammed. I was on the first day of the power tour I had ran almost 800 miles that day to get to the starting point I had just put in an a500 before I left that was really making the highway travel great

Garrett Ellison said
But along those lines, I take it you are using an MSD timing light for the multiple spark discharge?

I am using a new timing light that says it can do both it seems accurate no dial back I set the motor to 15° by setting the balancer and then locking down the rotor right on the terminal then cranked it without fuel and watched it with the timing light and read 15° on the balancer but when I tried to start it. it would not hit At all until I turn distributor way up 35 -40° it starts and idles doesn’t overheat I can Revit up but you can tell it’s missing a little bit when you raise the idle up because it’s too advanced but for some reason that’s the only way it will run. if I try to lower the timing back down while it’s running it will stumble and die