Ok back story I just installed a brand new 350 motor from advanced autos in my 89 Sierra engine runs pearl for a couple of days then all of a sudden boom my muffler blows up literally ok thought maybe I was running rich adjusted timing and runs great for another day then again boom so I'm thinking wtf now on the new motor I had to install the intake and throttle body so I checked for an intake leak replaced the distributor still runs good for a bit then blows up a muffler I don't smell gas so I'm not thinking throttle body rebuild I'm at a complete lose

 

The last time tho It blew it ran like **** disconnected the computer like u would do setting the timing and it runs better no idea on blowing a muffler that was two hours ago

 

Ur engine is running rich and filling the muffler with raw gas. One spark from anywhere and BOOM!