Mercruiser 3.0 cutting out in gear?

Fuel injection systems create more heat as the fuel is pumped to a higher pressure so hence the coolers on the engines.
( just like many high pressure electronic common rail diesels also now require coolers)
Many car fuel injections are moving away from return lines as I guess its cheaper, not sure?
Corvettes have not had return line systems for several years.
If a pressure line leaks in a car it drips on the ground.
Think where a return line leak is going in a boat!!
The high pressure pump in a stern drive must be at the engine and an outboard on the engine. USCG is just trying to avoid potential problems.

Vapour lock has never been a problem from design, only from poor maintenance in a boat. Engine box temp on a boat is way lower then a car and no exhaust system running down the chassis close to fuel lines under low pressure.
Poor fuel adds to the problem
Vapour lock/ plain old restrction on a carby engine is 99% caused by the anti siphon valve being faulty or some other restriction as mentioned by Gonzo.
If no part of your fuel lines are below the max fuel level you can remove it.
Many tanks have them as standard from the supplier and the boat builder just installs the tanks therefore many rigs do not need them.
It looks just like a std 3/8 hose barb although maybe slightly longer.
The mechanical pumps capacity is huge so if you can get the fuel to the pump there is no problem after that.

If water goes into one of those 2 barrel carbs you will need to remove the top of the carb and clean it out as it will corrode and the main jets and the powervalve are in the bottom right where the corrosion is.
Depending what age your 3.0 is there might be a filter on the fuel pump as well. Post a photo of your pump.

 

Not quite true Gonzo, look at this:
http://www.boatfix.com/merc/Servmanl/16/16A5R2.PDF next to the Weber 4B diagram.

It strikes me as odd that USCG allows a 10 cents plastic hose along a hot engine block from the fuel pump and the carb, but forbids a copper return line.

 

CDK
The hose is the vent line ( they call it a sight tube as when there is liquid in it change the fuel pump) between the fuel pump diaphrams ( on twin diphram pumps) or under them to vent the non fuel side of the diaphram as they are sealed from the outside and the sump so any failure will go up the little green pipe to the carb.
That would have been tested by the Underwriters Laboratory so if the USCG ever had wind that it was not fool proof it would have been re engineered years ago.
All Marine carburettors are vented under the flame arrester only.

Copper is a no no as it works hardens and fails when subjected to vibration.

 

Sorry Powerabout, I disagree.
Vapour lock has always been a problem in all designs with a single fuel line. Not when the engine is running, but to start it on a hot day.
Any gasoline engine with a carburetor is hard to start because the fuel in the float chamber has partly evaporated and the pump body contains only vapour.

Engine box temp on a boat at rest is higher than a car due to the lack of ventilation. All the boats I owned and were quite willing to start in the North Sea area presented problems in the Mediterranean afternoon. Both Merc, OMC and Volvo Penta needed several attempts and sometimes lifting the cover and waiting a few minutes was the only way to get things in motion.

And I still observe the same here every day during the tourist season. These people stay for an hour or so to buy groceries or drink a beer and by the time their engines finally start the gasoline odor has reached my terrace.

 

Here is a picture of one of the last Mercs I "consumed", just before the poor Ebay buyer came to collect it.
You can see the cheap plastic tube running from the pump cover along the white oil filter, then surfacing again under the flame arrestor.

They may call it a sight tube, but there is nothing to see there: it is partly painted black, the rest is yellowish brown (and brittle).
I am familiar with the story that it is a safety feature, but look where is it connected to the fuel pump, not between diaphragms but on top of them. Isn't that going through the motions instead of serious engineering?

The boat came with copper fuel lines all the way from the tank top to the engine mounts, where a short braided hose connected it to the fuel pumps.

 

Fair enough
Fuel injection is the answer
but....
I dont think the fuel can escape the fuel pump as it would either have to drain back to the tank ( the valves should stop this and the pump is usually close to the fuel level) or go up to the carb and flood it, which it should also not do if the needle and seat is in good condition.
I would say that most hard starting after a hot soak is a too rich condition from the evaporation as you mentioned and or the fuel trying to expand between the fuel pump and carb during the hot soak and going the easiest route which is usually to overcome the needle and seat, raise the fuel level and flood into the inlet manifold.
I guarantee you on a hot day if you remove the flame arrester and operate the throttle the accelerator pump will pump fuel as it draws from the bottom of the float bowl and is always full at rest. You would have to evaporate about 100ml of fuel to empty the bowl and as soon as the level goes down the fuel trapped under pressure behind the needle and seat will rush in as the fuel pump diaphram slowly relaxes so hence even more fuel to evaporate.

Vapour lock in a car is from between the inlet to the fuel pump and tank becoming so hot the fuel vaporises and hence the pump cannot pump the vapour so the carb runs out of fuel and that can happen whilst you are driving

 

CDK
Yes the hose is over the diaphram on your model but it is referred to as below the diaphram as it is not on the fuel side.On your model the pump is mounted such that the fuel is the lower part and the air cavity where the arm goes to the cam is on the top. So yes it is actually above it on your engine
Earlier models the pump was the other way up with the filter on top so hence I used the term under the diaphram.
Inlines also usually only had a single diaphram pump.
V8's also had both designs

 

We are in complete agreement here.