Okay - that prop sounds like it's about right for the size and weight of the boat. But again, these engines need to be set up so that WOT is in the 5800 to 6000 rpm range. Hopefully, the folks that set it up did it correctly. Still, if you can readily get your hands on a prop with a lower pitch, say, 15", it may be worth experimenting.
The first thing I would do is to vary the trim to see if you can increase the rpm's. Next, with the engine a mid-trim check to see if the anti-cavitation plate is just about exactly level with the lowest point on the transom. (The anti-cavitation plate is that wide piece of metal just above the prop.)
The VST is in place to assure an adequate amount of non-turbulent fuel is available to the high pressure fuel pump. (The fuel in the VST tank is kept under pressure to prevent vaporization, thus preventing the fuel from becoming too lean under variable demand.) On the back starboard side of your motor, you will see a plastic tube looped around a holder. Unhook it, and look down at the base where it connects. Beside the connection is a small set screw. Use a long 10” or longer, flat blade screwdriver to open the set screw - slowly. First, turn on your key switch briefly so that the HP fuel pump charges the system. Then, with the end of the hose extended downward into a clear container that will hold at least a quart of liquid, start opening the set screw. Do not open too far - if it comes out, you will need to remove the lower cowling to retrieve it. Somewhere around two pints of gas, and maybe some water and gunk will come out of that tube. Be patient, the last stuff to come out is usually slow, but drain out as much as you can. If there is water, or any gunk, then your fuel-water separator and low pressure fuel filter have failed and need to be replaced. If there is no flow, blow low pressure compressed air back through the tube to open it up.
If you are still having problems, then it may be necessary to remove the VST and clean the screen on the outflow tube and check the internal float valve.
The first thing I would do is to vary the trim to see if you can increase the rpm's. Next, with the engine a mid-trim check to see if the anti-cavitation plate is just about exactly level with the lowest point on the transom. (The anti-cavitation plate is that wide piece of metal just above the prop.)
The VST is in place to assure an adequate amount of non-turbulent fuel is available to the high pressure fuel pump. (The fuel in the VST tank is kept under pressure to prevent vaporization, thus preventing the fuel from becoming too lean under variable demand.) On the back starboard side of your motor, you will see a plastic tube looped around a holder. Unhook it, and look down at the base where it connects. Beside the connection is a small set screw. Use a long 10” or longer, flat blade screwdriver to open the set screw - slowly. First, turn on your key switch briefly so that the HP fuel pump charges the system. Then, with the end of the hose extended downward into a clear container that will hold at least a quart of liquid, start opening the set screw. Do not open too far - if it comes out, you will need to remove the lower cowling to retrieve it. Somewhere around two pints of gas, and maybe some water and gunk will come out of that tube. Be patient, the last stuff to come out is usually slow, but drain out as much as you can. If there is water, or any gunk, then your fuel-water separator and low pressure fuel filter have failed and need to be replaced. If there is no flow, blow low pressure compressed air back through the tube to open it up.
If you are still having problems, then it may be necessary to remove the VST and clean the screen on the outflow tube and check the internal float valve.
