I have what could be called a new 18' 1972 Sidewinder jet boat. The pump is a 12-JC A impeller. All new rebuild ( done myself last winter). The engine is a 455 bored .060 over ( Marshall Marine rebuilt longblock). Factory intake, Holley Marine 750 dual feed carb 9015 model (not doubble pumper). jets are currently 73's in front and stock whatever in rear bowl. That has a funny looking screw in it to remove the metering . So I left it alone. HEI distributor with an MSD processor in it for rev limiting. Log exhaust. Place hydraulic diverter. I believe the loader is an SS loader. It is the smaller loader for lower horsepower engines. I dont see any hook in the hull when I check it. The bunk boards exit the rear of the boat when it is on the trailer and I have no cavitation or porpise in it. the only problem I'm having is that I cant get the boat over 4000 RPM. At 4000 rpm though it running somewhere between 55 and 62 MPH ( guessing without GPS). It's got plenty of throttle left. I've got about 20 degrees of throttle throw left. the extra throttle just doesnt get it anywhere as far as performance pickup. I only use that part of the throttle to kick the secondaries in. My question is this. At which end do I look for the performance? Impeller or engine. I don't know what a jet should do as far as the overall performance power band, but this thing will rip right out of the hole. Last year the jet was so sluggish we would have to run about 3000 rpm to get enough movement out of the boat on the lake to move through rougher water. This year we can maneuver to a channel at 1500-1800 rpm. We just cant get to 4500 where I'd like to be. Any thoughts?