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Before people jump in here saying it's the CPS (rpm sensor), which is a common cause of no-starts, your '89 780 with B230FT should be LH 2.2 with EZ-117K ignition that uses a Hall Effect sensor for timing advance and rpm sensing. Only the B230F NA switched to LH 2.4 in 1989, along with the newly introduced DOHC B234F. The B230FT became LH 2.4 in the next model year.
IMO, it likely isn't the idle air valve or block temp sensor. The idle air valve controls idle rpm by altering the amount of intake air bypassing the almost closed throttle plate. It's all metered air (comes from the AMM) and doesn't affect mix or combustion, just rpm. A bad block temp sensor usually doesn't affect starting, but typically causes high idle rpm and rough, rich running.
If you're sure it's excess fuel, as in unburned fuel, then my first guess for a badly intermittent no-start would be the ignition side, not the fuel side.
Does the tach needle jump during cranking? If not then likely the Hall sensor. Have you checked for spark? Note if a strong or weak spark or if only one plug sparks. Prime suspects will likely end up being the distributor (badly worn cap/rotor or more likely a bad Hall sensor) followed by ignition wiring (most likely coil to distributor).
To lead you through the ignition side diagnosis, read the FAQ here
https://www.brickboard.com/FAQ/700-900/ElectricalIgnition.htm#CarWontStartIgnitionDiagnosis
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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