Exact symptoms with K jetronic on an 85 740. Most likely explanation seems to be fuel pressure regulator. Timing light confirms spark on number one cylinder so Hall sensor and timing is all OK. Plugs are wet with fuel and a squirt of fuel into the manifold gets no reaction so it is not starving for fuel. Confusing symptom is pressure in the inlet manifold blowing off hoses which suggested timing was off or valve stuck but all cylinders are 150 psi. Fuel pressure regulator is easier to replace on 740's with electronic injectors. Have not tried replacing the FPR on the K-jetronic yet but it seems the most likely culprit. I still don't understand how faulty fpr gives symptom of bad timing.