First thing I'd look for is a vacuum leak. Lots of old rubber plumbing in there. A smoke test will be the most efficient way of doing this, but the M20 is not as packed in there as more modern motors, so you may be able to find it without.
Failing that, test fuel flow and pressure per Bentley (you do have a Bentley manual, right?). I can't recall if the M20 has a cold start injector like the early M30s did; if it does, pull it out and see if it's leaking.
There are other possibilities, but let's eliminate the basics first.
Life's tough. It's tougher when you're stupid. -John Wayne
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