After almost 40 years, it is probably best just to replace it with a new, safe furnace.
If you want to do a total disassembly, look for mud dauber or wasp nests in the intake system, pipes, fan itself, or other areas. Check the rubber crossover tube, it was a failure point on those furnaces and is not available, recall was over years ago. The replacement tubes had a red stripe on them.
If all looks clean then check for rust in the main cast iron burner. Lots of disassembly needed to get there. Rust can block the slots and make for lazy and off colored flame.
But, basically, replace the furnace. Your life is not worth the risk, and many have leaking combustion chambers as well as a host of other issues.