1976 Cadillac El Dorado Exhaust manifold glows red hot

Hello. I have problem with 76 caddy edlorado. Car is very old and wasn't driving for last few years. I got this car few months ago but today I had time and check what condition is really this car. From last owner I heared the transmission will needs repair and that's all. I checked angine oil , put new gas to the gas tank. Engine starts without any problems but after about 3-4 minutes I saw intake manifold be glowing red and hot. I turn off the engine. I am looking for any help with find a reason of glowing. What should I do at first? I will be very thankful for help. Thank You.