I'd check the hose going to the 50 mph transmission lock up modulator first as it sounds like the likely culprit.
show me vacuum diagram on 1976 malibu
bad vacuum hoses or leaking intake manifold gaskets.
800rpm manual transmission, o degrees advance 1200rpm automatic transmission 4 degrees advance
Might be a vacuum leak
don gory. me help yu
google images
I have a 2000 Malibu, I can help you out if you can give me some more info...where on the intake manifold are you referring to?
because vacuum will make the glass collapse
it is at end of vacuum hose, on top of valve cover..
Check all your vacuum hoses. Also change your fuel filter.
If you have power brakes look where the large hose that goes to the brake booster connects to the engine and that is your main vacuum source.
The vehicle does this on its own with a vacuum operated fuel pressure regulator.