I don't see an issue for just idling. Mine was running a bit rich and resemble mine with VST

Just a thought......I was going through my engines and changing fuel filters this past weekend. On my weaker motor, I found where the Mercruiser factory fuel line (one that runs from filter head to fuel pump) has collapsed about 1/2" past the stainless factory crimped fitting. I have seen that happen on other motors, especially if someone had messed with lines and not been careful with their placement or not holding the fitting bend to prevent rotation while tightening.

You might want to inspect those hoses between separator/filter and VST pump. Would give a major supply restriction if collapsed somewhere.

Have you tried running it on a separate gas container to rule out a supply/ tank issue, does it have separate fuel tanks for each motor? Your other motor is running good if i remember.

Same tank for both engines and the other is running fine. The suspect pickup has been checked and is clean, the supply line to lift pump is new, the 90* fitting at the lift pump is clear, the lift pump is new, the VST rebuilt including new electric pump, fuel pressure regulator new, all associated vac lines and vent lines checked and clear. Literally the only part of the fuel system that hasn't been touched is the injectors but per the spark plug pics posted above I don't see a glaring issue with an injector.

Keith

On my 502 there is a yellow vent tube that lead down to the mech fuel pump, my understanding was this was only in case the mechinal pump diaphragm failed it would send fuel to the vst instead of puking it into the bilge. There was a bigger shorter black hose that vented to the plenum from the vst had issues and would send it to the intake near cylinders # 5&7. Just wanted to verify you checked this vent while running.

All yellow hoses new and clear of fluids.

VST to plenum hose verified dry as a bone while running 5 days ago.

Small filter on the pressure regulator clogged? Anti-siphon valve on tank?

Pressure regulator brand new Mercruiser and valve in tank eliminated years ago.

UPDATE

With the adjustable fuel pressure regulator it idled cold beautiful yesterday with base pressure at 36 psi, corrected vacuum at 28 psi. Both are within manual spec. Prior to the adjustable FPR the (vacuum) fuel pressure was at 26 and would constantly stall cold.

Once it began warming up the IAC steps got higher and it was stalling. IAC steps were 20's when cold and gradually got to 70's when hot. No other parameters on the scanner/logger changed. I don't understand why the IAC is closing and choking off air. It's a brand new Mercruiser unit, the 4th I've tried now. There was no change in gauged vacuum nor fuel pressure. Ran out of time but I'm going to re-check timing next as that's literally the last thing I can do, other than pulling injectors and sending out for testing.

Keith