Rich,
It could be electrical, as Goldie says but if it consistently runs for roughly the same length of time then it could be a fuel starvation issue. Check the fuel line, all the jets and airways in the carb as you may have some build-up of 'varnish' from the old fuel. Unless you know that everything was drained out in 1974. Some more symptoms might make the 'e-diagnosis' a bit easier. Cheers.