riverboater, are your motors rebuilt now? And are you still trying to use the VST's on them? You asked about removing the mechanical FP at the sea pump and installing an electric lift pump on another thread, so I'm just trying to see where you are at in the whole project.

Yes having motors done now
Builder said to install check valve in water line to keep water in motor
Just trying to cover all bases Vapor lock was not good last year it sucked

http://www.boatfix.com/merc/Bullet/99/99_07.pdf

I've read all thread about vapor lockingi. I had same symptom on my 1996 mercruiser 454 MPI so took apart vst and the spring was under the plate keeping a vent closed. I want to place spring above the plate like you did but my concern is that vent will be opened all times due to the spring pushing the plate with the rubber on top of it down off the vent opening.
Is that ok?
I'm afraid that vaporized gas will be gathering in the engine compartment which can be dangerous.

The vent hose SHOULD run into the throttle body, but that only helps when the engine is running.

I feel like I could write a tech bulletin on this and having just battled it, here's what I'd do if starting all over-

Check the fuel rail pressure, you need 34-38psi with the engine off, 3-10psi lower with the engine idling, and 37 with the engine running above idle.
If you're low, block the return line. If the pump makes 60psi or more, the pump is good and the reg is bad. If you block the return and the pressure stays constant, the pump is bad.

That's basically a free fuel pump check, after it all pans out you need to figure out where the fuel is vaporizing and cool it off. Vapor lock is a function of temperature and pressure, and pulling the fuel around a hot block is not helping anything.
Try first to reroute the fuel line to the VST away from the block, and the send the return back to the tank. If that doesn't do it, a new pump mounted as low in the transom as possible, along with the aforementioned mods, should cure the issue.

You are talking about sending return to the tank. Return from where? VST?

The return from the rail, it is the bottom hose at the back of the fuel rail.

It depends on if you have the new style or old, but we'll assume you have the newer style with the clear hose connected to the top. If the diaphram is not working then it may not be venting correctly. Although routing the return line to the tank will do a lot to fix the problem, if the VST tank isn't venting when the motor is running it will still not work right. With your top off the vent should be closed off. When you put a vacuum on the hose that comes from the manifold vacuum hose, it should pull the diaphram open to vent the gas vapor through the clear line to the throttle body. If the diphram is faulty or leaking then it will not open to vent the VST.
Capt, if you place the spring on the top of the diaphram and screw the diaphram housing on, then try to blow through that top vent tube it should not pass air. Give that a try and see if you can blow through it (no need to put the top back on the tank to test this) If you had a hand held vacuum pump connected to the side port and pumped up vacuum then it should open the diaphram valve and allow you to blow through the vent port. I really need to scan that rebuild section from the manual and post it here and maybe I will get teh book out this weekend. Let me know what you find.

If I do that won't it depressurize the whole fuel rail and inhibit the flow of fuel to injectors.

Yes, need a regulator in there