Actually I had given some thought to the possibility of the timing being out of whack. That’s why I asked if it had recently been worked on.
Being a 1990 740 I am assuming you have the cam-driven distributor rather than the side mounted intermediate shaft-driven version.
First off, you have verified that the rotor is turning so you know the timing belt must still be in tact, but you don’t know if it may have jumped a few teeth.
Normally you could use the timing marks on the crank pulley to verify TDC but, unfortunately, the 740s use a two piece pulley that consists of an inner section that’s keyed to the crankshaft and an outer ring that carries the accessory belt, and those two sections are isolated from each other with a rubber filler. With old age the rubber starts to loosen and the outer pulley ring can spin on the inner section, causing the timing marks unable to accurately indicate crankshaft angle. You can use a plastic straw inserted in the #1 spark plug hole to “feel” when the piston is at TDC. You may be a degree or two off but you will be close enough to TDC for this exercise. Now look at the crank pulley timing mark and see if it’s at, or very very close to, TDC. If it’s at TDC then you know the pulley has, most likely, not spun and you can now rely on it for an accurate crankshaft angle reading.
Remove the upper timing belt cover and see if the cam sprocket timing mark aligns with the index mark inside the rear timing belt cover. If it’s nowhere close then turn the crank one complete revolution and recheck the cam sprocket timing mark alignment. If it still doesn’t align then something somewhere has slipped - possibly a loose belt, stripped belt teeth, or sheared woodruff key between the crank pulley and the crankshaft.
You will also want to check the relationship between cam timing marks and the rotor position. When the cam and crankshaft are at TDC the rotor should be pointing to the #1 distributor cap contact. If it doesn’t match up then it’s possible the cam sprocket-to-cam keying has failed.