I would second the "reverse the hose" test as a first off - the arrows on these bulbs usually point in the direction of flow. If it's more convenient to have the bulb at the engine end then you can turn it round easily (I assume two jubiulee clips are holding the hose onto the fir trees) - remember at idle it will be using next to no fuel, and so the ball valves in the primer will probably not seat hard (or at all), which may explain why it will run OK at idle.
Yes, no load open throttle will need less fuel than when under load, and I'll bet you probably don't hold it at high RPM for long when you do that due to the antisocial nose, also most engines of that age limit throttle opening to about half when not in gear, so chances are you wonl't have built up enough vaccuum in the line to stop it by the time you are deafened by it and throttle back!
When you say it dies, does it "fade out" or does it just stop? Assuming a "fade", and you have no success with the reversed hose, a good test as to whether you are running lean or not is give the choke a quick poke as it starts to die (I assume it's remote control and you have a choke button, if not a quick "out-in" of the choke lever will have the same effect)- one of two things will happen- if it takes off like a scalded cat, your mixture is slightly too lean, if it coughs & dies you are too rich. A mommentary hesitation and it's probably OK. Engines are remarkably fussy about the air - fuel mix, both too lean or rich will kill it. Increase load (i.e accelerate in gear) and the "perfect balance" you had before will suddenly not be enough. No idea about to the carbs on that engine, but there will likely be an idle mix adjuster somewhere in there, which will probablky affect things up to around the 1500/ 2000 rpm region.....
I guess you have an SR4 - with the water ballast, the engine load to get up on the plane is quite phenomenal, and what is needed to push it along in displacement mode or even on the plane is nothing compared to what it needs to empty the ballast & lift the hull up to plane (try driving one with a 25 on the transom & you'll see what I mean!) Mine suffered a similar problem - I got round it by poking the choke on accel for a day before I got RSI of my thumb and adjusted things) But yes, the fuel consumption at load is a LOT more than at low / no load for a given RPM. (Think of your car with and without the trailer / roof rack)
Hope this helps. Not sure where you are in the Clyde, but I may be out this weekend if I can be of any assistance. Problem is at the moment I don't know whether I'll be free Sat or Sun yet.